Piezoelectric composites comprising covalently bonded piezoelectric particles and use thereof in additive manufacturing

ABSTRACT

Parts made by additive manufacturing are often structural in nature, rather than having functional properties conveyed by a polymer or other component present therein. Printed parts having piezoelectric properties may be formed using compositions comprising a polymer material comprising at least one thermoplastic polymer, and a plurality of piezoelectric covalently bonded to the at least one thermoplastic polymer and dispersed in at least a portion of the polymer material. The compositions are extrudable and may be pre-formed into a form factor suitable for extrusion. Additive manufacturing processes using the compositions may comprise forming a printed part by depositing the compositions layer-by-layer.

FIELD

The present disclosure generally relates to additive manufacturing and, more particularly, extrudable compositions suitable for additive manufacturing to form printed parts exhibiting piezoelectric properties.

BACKGROUND

Additive manufacturing, also known as three-dimensional (3-D) printing, is a rapidly growing technology area. Although additive manufacturing has traditionally been used for rapid prototyping activities, this technique is being increasingly employed for producing commercial and industrial parts in any number of complex shapes. Additive manufacturing processes typically operate by building an object (part) layer-by-layer, for example, by 1) depositing a stream of molten printing material obtained from a continuous filament or other printing material source, 2) sintering powder particulates of a printing material using a laser, or 3) direct writing using an extrudable paste composition. The layer-by-layer deposition usually takes place under control of a computer to deposit the printing material in precise locations based upon a digital three-dimensional “blueprint” of the part to be manufactured, with consolidation of the printing material often taking place in conjunction with deposition to form the printed part. The printing material forming the body of a printed part may be referred to as a “build material” herein.

Additive manufacturing processes employing a stream of molten printing material for part formation may utilize a thermoplastic polymer filament as a source of the molten printing material. Such additive manufacturing processes are sometimes referred to as “fused deposition modeling” or “fused filament fabrication” processes. The latter term is used herein. Additive manufacturing processes employing thermoplastic polymer pellets or other polymer forms as a source of printing material are also known. Extrudable paste compositions comprising thermoplastic polymers or curable polymer precursors (resins) may also be utilized in similar direct writing additive manufacturing processes.

Additive manufacturing processes employing powder particulates of a printing material oftentimes perform directed heating in selected locations of a particulate bed (powder bed) following printing material deposition to promote coalescence of the powder particulates into a consolidated part. Techniques suitable for promoting consolidation of powder particulates to form a consolidated part include, for example, Powder Bed Fusion (PBF), selective laser sintering (SLS), Electron Beam Melting (EBM), Binder Jetting and Multi-Jet Fusion (MJF).

A wide range of parts having various shapes may be fabricated using the foregoing additive manufacturing processes. In many instances, build materials employed in such additive manufacturing processes may be largely structural in nature, rather than the polymer having an innate functionality itself. One exception is piezoelectric functionality, which may be exhibited in printed objects formed from polyvinylidene fluoride, a polymer which possesses innate piezoelectric properties upon poling. Piezoelectric materials generate charge under mechanical strain or, conversely, undergo mechanical strain when a potential is applied thereto. Potential applications for piezoelectric materials include, for example, sensing, switching, actuation, and energy harvesting.

Despite the desirability of forming printed parts having piezoelectric properties, there are only limited options for doing so at present. Other than polyvinylidene fluoride, the range of piezoelectric polymers is rather limited, and some alternative polymers are not suitable for being printed in additive manufacturing processes employing extrusion. For example, crosslinked polymers are completely unworkable once they have been crosslinked, and polymer resins suitable for forming crosslinked polymers may not by themselves afford form factors suitable for printing in fused filament fabrication and similar printing processes and/or printed parts formed from polymer resins may not be self-supporting before crosslinking takes place. Moreover, the piezoelectricity of polyvinylidene fluoride is rather low compared to other types of piezoelectric materials. These shortcomings may limit the range of printed parts having a piezoelectric response that may be obtained through present additive manufacturing processes.

Numerous ceramic materials having high piezoelectricity are available, such as lead-zirconium-titanate (PZT), but they are not printable by themselves and are often very brittle. Moreover, high sintering temperatures (>300° C.) may be needed to promote part consolidation and piezoelectric particle interconnectivity after depositing predominantly a piezoelectric ceramic. Admixtures of polymers and piezoelectric particles have not yet afforded high piezoelectric performance in printed parts. Poor dispersion of the piezoelectric particles in the polymer, particle agglomeration, and limited interactions between the piezoelectric particles and the polymer are to blame in many instances. Without being bound by any theory, the limited interactions between the piezoelectric particles and the polymer results in poor load transfer to the piezoelectric particles, thereby lowering the piezoelectric response obtained therefrom when mechanical strain is applied. Particle agglomeration may also play a role in this regard.

SUMMARY

In some embodiments, the present disclosure provides compositions comprising a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles covalently bonded to the at least one thermoplastic polymer and dispersed in at least a portion of the polymer material; wherein the composition is extrudable. Printed parts may comprise the compositions.

In other various embodiments, the present disclosure provides compositions comprising: a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles dispersed in at least a portion of the polymer material and reactive with the at least one thermoplastic polymer under specified conditions to form a plurality of covalent bonds; wherein the composition is extrudable. Printed parts may comprise the compositions.

In other various embodiments, additive manufacturing processes of the present disclosure may comprise: providing a composition of the present disclosure, and forming a printed part by depositing the composition layer-by-layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to one having ordinary skill in the art and having the benefit of this disclosure.

FIG. 1 shows a schematic of an illustrative fused filament fabrication process for producing a part using a build material and a removable support material.

FIG. 2 shows a schematic of an illustrative part having a first removable support interposed between the part and a print bed and a second removable support interposed between two portions of the part.

FIG. 3 is a diagram showing overlaid FTIR spectra of PZT particles and PZT-NH₂ particles.

FIG. 4 is a diagram showing overlaid FTIR spectra of HDPE-g-MA and the composite of Example 3A.

FIG. 5 is a diagram showing overlaid FTIR spectra of SEBS-g-MA and the composite of Example 6.

FIG. 6 is a diagram showing overlaid FTIR spectra of HDPE-g-MA and the composite of Example 4B.

FIGS. 7A-7C show illustrative SEM images for HDPE composites produced in Examples 2, 3A, and 4B, respectively.

FIGS. 8A and 8B show illustrative SEM images for SEBS composites produced in Examples 5 and 6.

DETAILED DESCRIPTION

The present disclosure generally relates to additive manufacturing and, more particularly, extrudable compositions suitable for additive manufacturing to form printed parts exhibiting piezoelectric properties. More specifically, the present disclosure provides compositions in which piezoelectric particles are combined with a polymer material in a composite having a form factor suitable for additive manufacturing, and in which piezoelectric particles are covalently bonded to at least one thermoplastic polymer comprising the polymer material. Other than being covalently bonded to the piezoelectric particles, the at least one thermoplastic polymer is otherwise non-crosslinked, thereby facilitating extrusion of the compositions. Alternatively, the covalent bond formation may occur latently, such that the piezoelectric particles and the at least one thermoplastic polymer react under specified conditions to produce a plurality of covalent bonds, which may occur during or after printing. The covalent bonding between the piezoelectric particles and the at least one thermoplastic polymer within the polymer material may increase the piezoelectric response (piezoelectricity) obtained from the composites and printed parts formed therefrom. The composites are extrudable and may have various form factors such as, but not limited to, composite filaments, composite pellets, composite powders, and composite pastes.

As discussed above, additive manufacturing processes, such as fused filament fabrication, direct writing, or similar layer-by-layer deposition processes, are powerful tools for generating printed parts in a wide range of complex shapes. In many instances, the polymer materials used in layer-by-layer additive manufacturing processes are largely structural in nature and do not convey functional properties to a printed part by themselves. Polyvinylidene fluoride is a notable exception, which may form printed parts having piezoelectricity after suitable poling. Beyond polyvinylidene fluoride, there are few alternative polymer materials for introducing piezoelectricity to a printed part. Furthermore, the piezoelectricity of polyvinylidene fluoride may not be sufficiently large for some intended applications.

In response to the foregoing shortcomings, the present disclosure provides compositions that are composites capable of undergoing extrusion to form printed parts through layer-by-layer additive manufacturing. The composites may comprise a polymer material and piezoelectric particles dispersed in at least a portion of the polymer material, wherein covalent bonding may occur between the piezoelectric particles and at least a portion of the polymer material. The composites may have more robust mechanical properties than do piezoelectric particles alone, at the least being less brittle and more flexible, and may be formed more readily into printed parts than can the piezoelectric particles alone. More specifically, the compositions described herein comprise a plurality of piezoelectric particles that are covalently bonded to (or reactive with under specified conditions) at least one thermoplastic polymer within a polymer material comprising the compositions. The covalent bonding between the piezoelectric particles and the at least one thermoplastic polymer may increase compatibility between the piezoelectric particles and the polymer material and enhance the piezoelectricity obtained from the composites once formed into a printed part. Without being bound by any theory or mechanism, the covalent bonding is believed to enhance the piezoelectric effect (piezoelectricity) by promoting load transfer from the polymer material to the piezoelectric particles, but without rendering the compositions into a non-extrudable form. To maintain the compositions in an extrudable form, the at least one thermoplastic polymer may remain non-crosslinked other than being covalently bonded to the piezoelectric particles. The at least one thermoplastic polymer may further include at least some thermoplastic polymers that are not covalently bonded to the piezoelectric particles as well, thereby further aiding extrudability. Evidence of covalent bonding between the at least one thermoplastic polymer and the piezoelectric particles may be demonstrated spectroscopically, as well as by imaging showing improved dispersion of the piezoelectric particles within the polymer material and/or enhancement of the piezoelectric response in comparison to that produced without covalent bonding being present. A variety of covalent bonding strategies may be suitable in regard to the foregoing and are described further below.

To improve the piezoelectric response still further, carbon nanomaterials may optionally be included in the compositions disclosed herein. The carbon nanomaterials may increase stiffness of the polymer material in a composite and further facilitate load transfer from the polymer material to the piezoelectric particles, thereby increasing the piezoelectric response obtained therefrom. In addition, some carbon nanomaterials possess significant electrical conductivity and may increase the piezoelectric response attainable when present in combination with a given quantity of piezoelectric particles. At the very least, carbon nanomaterials that are electrically conductive may improve the efficiency of the poling process used to induce piezoelectricity in a printed part, thereby further enhancing the piezoelectric response attained therefrom. Like the piezoelectric particles, the carbon nanomaterials also optionally may be covalently bonded to at least a portion of the polymer material, including the at least one thermoplastic polymer, and/or be crosslinkable with themselves and/or the piezoelectric particles to facilitate the foregoing still further. Illustrative carbon nanomaterials that may be suitably used in the disclosure herein are specified further below.

Suitable form factors of the composites that may be processed by extrusion in the disclosure herein include composite filaments, composite pellets, composite powders, composite pastes, or any combination thereof. Additional details regarding these various form factors follows herein. By virtue of the at least one thermoplastic polymer present therein, the compositions may remain extrudable once a composite with piezoelectric particles has been formed, thereby allowing printed parts to be formed directly through extrusion and solidification of the at least one thermoplastic polymer. Optionally, a polymer precursor, such as a polymerizable monomer or oligomer or a curable resin, may be present in combination with the at least one thermoplastic polymer in the polymer material. The term “curable resin” refers to a divalent polymerizable substance that undergoes covalent crosslinking upon being cured. After printing, a curable resin may form a stiff, covalently crosslinked polymer matrix within or interpenetrating the at least one thermoplastic polymer in non-crosslinked form, optionally with crosslinking also occurring between the crosslinked polymer matrix and the piezoelectric particles. Such polymer precursors may be utilized for polymers that would otherwise be too stiff or not easily extruded when blended with piezoelectric particles in a composite. Advantageously, the combination of at least one thermoplastic polymer having piezoelectric particles covalently bonded thereto (or reactive therewith to form covalent bonds under specified conditions) and a polymer precursor, such as a curable resin, may remain readily extrudable and allow modification of a printed part to take place after printing. Namely, after forming a printed part by extrusion and layer-by-layer deposition of a suitable form factor, the polymer precursor may be converted to the corresponding covalently crosslinked polymer. The stiffness of the resulting covalently crosslinked polymer matrix may again promote load transfer to the piezoelectric particles to increase the piezoelectric response obtained therefrom. The piezoelectric particles need not necessarily be dispersed in the covalently crosslinked polymer matrix for this benefit to be realized.

Advantageously, a range of thermoplastic polymers having functionality capable of forming covalent bonds with piezoelectric particles and optionally with carbon nanomaterials are commercially available or may be readily produced by modification of a parent polymer backbone. Moreover, covalent bonding may take place through surface functional groups natively present upon the piezoelectric particles, such as surface hydroxyl groups. Alternately, piezoelectric particles may be readily functionalized with a linker moiety containing a functional group capable of undergoing a reaction with a complementary functional group upon the at least one thermoplastic polymer to form a covalent bond between the two. In addition, covalent bond formation between the piezoelectric particles and the at least one thermoplastic polymer may occur latently. A range of covalent bonding types are suitable for use in the disclosure herein and may be envisioned by one having ordinary skill in the art. Example covalent bonding strategies are discussed in further detail below. In another example, the piezoelectric particles may contain a functional group (ligand) capable of coordinating a metal in piezoelectric particles by way of a coordinate covalent bond.

Composite filaments and composite pellets containing a polymer material comprising at least one thermoplastic polymer having piezoelectric particles covalently bonded thereto (or reactive therewith) may be obtained by melt blending. Polymer precursors, carbon nanomaterials, and other optional components may be selected and included in amounts that do not significantly alter the melt blending process. Through selection of the polymer material and the amount of piezoelectric particles and other components present therein, these form factors may be formed into printed parts via extrusion and layer-by-layer deposition, such as through fused filament fabrication processes in the case of composite filaments, to afford significant piezoelectricity, after poling of the printed part. Composite filaments that are suitable for fused filament fabrication may have diameters that are appropriate for the drive unit for a particular printing system (common filament diameters include 1.75 mm and 2.85 mm). Other properties that may determine if a composite filament is suitable for fused filament fabrication include the temperature required to extrude the filament, which should not be undesirably high. A suitable composite filament for fused filament fabrication may further minimize printing issues, such as oozing from the print nozzle or clogging of the print nozzle, which may be impacted by the overall viscosity of the composite at the printing temperature. In addition, composite filaments suitable for fused filament fabrication may afford printed parts that easily separate from a print bed, have sufficient mechanical strength once printed, and exhibit good interlayer adhesion. Additional characteristics of suitable composite filaments and other form factors are specified below.

Composite filaments and other form factors obtained by melt blending may include the piezoelectric particles dispersed within at least a portion of a polymer material comprising at least one thermoplastic polymer having functionality that is reactive with the piezoelectric particles to form a covalent bond thereto. By virtue of the covalent bonding, the piezoelectric particles may become incorporated within the at least one thermoplastic polymer and dispersed throughout at least a portion of the polymer material, such as a substantially uniform dispersion of the piezoelectric particles within the at least one thermoplastic polymer. A covalently crosslinked polymer formed from a polymer precursor within the polymer material, if present, may be disposed in the same location as the piezoelectric particles or in a different location, depending on whether the piezoelectric particles are further covalently crosslinked with the covalently crosslinked polymer. Similarly, carbon nanomaterials or other components may be located in the same location as the piezoelectric particles or in a different location. For example, if carbon nanomaterials or other components have different surface properties than the piezoelectric particles or are unable to form covalent bonds to the at least one thermoplastic polymer, the carbon nanomaterials or other components may localize in different locations, including in different polymers in the case where the polymer material comprises multiple polymers. For example, carbon nanomaterials or other components may localize in a thermoplastic polymer that is unable to form covalent bonds with the piezoelectric particles, such as a thermoplastic polymer that lacks functionality compatible with forming covalent bonds with the piezoelectric particles and is immiscible with the at least one thermoplastic polymer covalently bonded to the piezoelectric particles (or reactive with the piezoelectric particles).

Suitable melt blending processes may include melt mixing with stirring, followed by extrusion of the resulting melt blend, or direct blending via extrusion with a twin-screw extruder. The piezoelectric particles may or may not become covalently bonded to the at least one thermoplastic polymer during the melt blending process. Such melt blending processes followed by further extrusion, further aided by the covalent bond formation resulting therefrom, may afford a good distribution of the piezoelectric particles in the at least one thermoplastic polymer within the resulting composite and printed parts obtained therefrom. Cryo-milling, grinding or shredding before further extrusion of the composite may further facilitate the extrusion process and further promote distribution of the piezoelectric particles within the at least one thermoplastic polymer. Preferably, the melt blending processes may be conducted without the combination of the polymer material and the piezoelectric particles ever being exposed to a solvent together, which may otherwise result in trace organic solvents remaining in the composites following extrusion in some instances, and undesirably become incorporated within a printed part. Moreover, melt blending with little to no void formation in the composites may be realized even in the absence of surfactants and other surface compatibilizers, which otherwise may be detrimental to include in a printed part. Further surprisingly and advantageously, little or no agglomeration of the piezoelectric particles within the polymer material may occur following melt blending, which may desirably improve the piezoelectric properties obtained after poling. A uniform distribution of individual piezoelectric particles in the polymer material or a portion thereof may be realized in some instances, wherein the piezoelectric particles remain above a percolation threshold concentration within the polymer material. The piezoelectric particles may be considered above a percolation threshold concentration if the piezoelectric particles communicate with one another to generate a voltage when a mechanical load is being applied to the composites.

Advantageously, high loadings of piezoelectric particles may be tolerated in the composites described herein, while still maintaining extrudability and affording printed parts having high structural integrity and with the piezoelectric particles remaining in a substantially non-agglomerated and dispersed state following printing. Distribution of the piezoelectric particles as individuals rather than as agglomerates may afford a significant increase in the piezoelectric response obtained after poling, since there may be a greater particle surface area available to undergo interaction with the polymer material and promote load transfer in between. This effect may be further supplemented through the covalent bonding between the piezoelectric particles and the at least one thermoplastic polymer according to the disclosure herein.

Composite filaments compatible with fused filament fabrication may be formed in the disclosure herein. Although composite filaments may be an advantageous and particularly versatile form factor, it is to be realized that composite pellets may also be produced through melt blending processes and used in similar additive manufacturing processes. Namely, a polymer material comprising at least one thermoplastic polymer and piezoelectric particles may be combined with one another under melt blending conditions suitable to promote covalent bond formation between the two, and instead of extruding to form composite filaments, larger extrudates may be produced, which may then be cut, shredded, pulverized, or the like to afford composite pellets of a specified size and geometry or composite powders having even smaller dimensions and a wide distribution of particle sizes. Other than having a different shape, the microscopic morphology of the composite pellets may be similar to that of composite filaments. Like composite filaments, composite pellets and composite powders may be subsequently processed into printed parts having piezoelectric properties under suitable additive manufacturing conditions.

In the disclosure herein, “filaments” are to be distinguished from “fibers” on the basis that filaments comprise a single elongate form factor, whereas fibers comprise multiple filaments twisted together (bundled) to form a fine thread or wire in which the individual filaments remain identifiable. As such, filaments have smaller diameters than do fiber bundles formed therefrom, assuming no filament compression takes place when forming a fiber bundle. Filaments obtained by solution electrospinning or melt electrospinning are usually up to about 100 μm in diameter, which is too small to be effectively printed using fused filament fabrication. The composite filaments obtained by melt blending and extrusion in the disclosure herein, in contrast, may be about 0.5 mm or more in size and dimensioned for compatibility with a particular printing system for fused filament fabrication.

Another suitable form factor that may be produced in the disclosure herein is an extrudable composite paste. As used herein, the term “paste” refers to a composition that is at least partially fluid at a temperature of interest. The term “paste” does not necessarily imply an adhesive function of any type. Moreover, the terms “paste” and “ink” may be used interchangeably with one another in the disclosure herein with respect to direct writing additive manufacturing processes. Unlike composite filaments and composite pellets discussed in brief above, extrudable composite pastes may comprise at least one solvent to facilitate extrusion. The polymer material may be at least partially dissolved or suspended in the at least one solvent, along with suspended piezoelectric particles, with covalent bonding to the at least one thermoplastic polymer occurring upon formulation or printing of the extrudable composite paste. Alternately, a polymer material and piezoelectric particles covalently bonded to (or reactive with) at least one thermoplastic polymer may be pre-processed into composite particles (e.g., by melt blending and decreasing the particle size) that are then suspended or emulsified together in the solvent. The at least one solvent may or may not dissolve the polymer material present therein. Optionally, suitable composite pastes may be at least biphasic and contain at least two immiscible fluid phases, wherein the polymer material containing covalently bonded piezoelectric particles are present in one or both of the at least two immiscible fluid phases. Localization of at least the piezoelectric particles in one phase or at an interface between polymer phases may increase the piezoelectric response attainable therefrom, since the effective concentration of the piezoelectric particles is increased. Additional details regarding extrudable composite pastes are also provided hereinbelow.

Before addressing various aspects of the present disclosure in further detail, a brief discussion of additive manufacturing processes, particularly fused filament fabrication processes, will first be provided so that the features of the present disclosure can be better understood. FIG. 1 shows a schematic of an illustrative fused filament fabrication process for producing a part using a build material and a removable support material. As shown in FIG. 1 , print head 100 includes first extruder 102 a and second extruder 102 b, which are each configured to receive a filamentous printing material. Specifically, first extruder 102 a is configured to receive first filament 104 a from first payout reel 106 a and provide molten stream 108 a of a first printing material, and second extruder 102 b is configured to receive second filament 104 b from second payout reel 106 b and provide molten stream 108 b of a second printing material. Both molten streams are initially deposited upon a print bed (not shown in FIG. 1 ) to promote layer-by-layer growth of supported part 120. The first printing material (build material) supplied by first extruder 102 a may be a piezoelectric composite of the present disclosure used to fabricate part 110, and the second printing material (removable support material) supplied by second extruder 102 b may be a dissolvable or degradable polymer, which is used to fabricate removable support 112 under overhang 114. Overhang 114 is not in direct contact with the print bed or a lower printed layer formed from the build material. Overhang 114 need not necessarily be present in a given printed part. In the part arrangement shown in FIG. 1 , removable support 112 is interposed between overhang 114 and the print bed, but it is to be appreciated that in alternatively configured parts, removable support 114 may be interposed between two or more portions of part 110. FIG. 2 , for example, shows illustrative part 200, in which removable support 202 is interposed between an overhang defined between part 200 and print bed 204, and removable support 206 is interposed between two portions of part 200.

Referring again to FIG. 1 , once printing of part 110 and removable support 112 is complete, supported part 120 may be subjected to support removal conditions 125 that result in elimination of removable support 112 (e.g., dissolution or disintegration conditions, or the like) and leave part 110 with overhang 114 unsupported thereon. Support removal conditions 125 may include contact of supported part 120 with a solvent in which removable support 112 is dissolvable or degradable and part 110 is not.

If a printed part is being formed without an overhang or similar feature, it is not necessary to utilize a removable support material during fabrication of the printed part. Similarly, two or more different build materials may be utilized as well, such as when one or more of the build materials is structural in nature and one or more of the build materials is functional in nature. In non-limiting examples, a structural polymer may be concurrently printed with a piezoelectric composite of the present disclosure. Further disclosure directed to such piezoelectric composites is provided herein.

Compositions of the present disclosure may comprise a polymer material comprising at least one thermoplastic polymer, and plurality of piezoelectric particles covalently bonded to the at least one thermoplastic polymer and dispersed in at least a portion of the polymer material. The compositions are extrudable and in a form factor suitable for extrusion. The at least one thermoplastic polymer is non-crosslinked other than being covalently bonded to the plurality of piezoelectric particles. Components within the compositions and amounts thereof may be selected such that the compositions remain extrudable. Examples of suitable piezoelectric particles and thermoplastic polymers effective for undergoing covalent bond formation are provided below. The piezoelectric particles may be dispersed within the polymer material in a uniform or non-uniform manner, such as a gradient distribution. In addition, at least a portion of the polymer material may not contain piezoelectric particles at all, such that the piezoelectric particles are concentrated in a particular polymer phase.

The compositions of the present disclosure may alternately comprise piezoelectric particles that latently form covalent bonds with the thermoplastic polymer. Such compositions may comprise a polymer material comprising at least one thermoplastic polymer, and a plurality of piezoelectric particles dispersed in at least a portion of the polymer material and reactive with the at least one thermoplastic polymer under specified conditions (e.g., temperature, catalyst, etc.) to form a plurality of covalent bonds. The at least one thermoplastic polymer may again remain non-crosslinked after being reacted with the piezoelectric particles to form the plurality of covalent bonds.

The compositions disclosed herein are extrudable and maintain the ability to form self-standing three-dimensional structures once extruded during an additive manufacturing process. The term “self-standing” means that a printed part holds its shape and/or exhibits a yield stress once the composition has been extruded into a desired shape. In contrast, compositions that do not hold their shape following extrusion are referred to as “conformal,” since they may assume the profile of the surface upon which they are deposited. In many instances, the ability for a composite to be extruded and the ability for the composite to provide a self-standing structure following extrusion are mutually exclusive features. For example, a composite that is extrudable may lack sufficient mechanical strength to support itself upon being deposited in a desired shape, and a composite that hold its shape within a three-dimensional structure may be too rigid to be extruded. The composites described herein may further be processed into various form factors capable of undergoing continuous extrusion.

The term “extrusion” and various grammatical forms thereof refers to the ability of a fluid to be dispensed through a small nozzle. In addition to producing self-standing structures, the composites disclosed herein may be formulated to maintain extrudability once they are heated at or above a melting point or softening temperature of a thermoplastic polymer therein. Both the thermoplastic polymer and the piezoelectric particles, as well as amounts thereof, may be selected to convey extrudability to the composites described herein. Composite pastes containing a thermoplastic polymer need not necessarily be heated at or above the melting point or softening temperature to facilitate extrusion, since such compositions are already at least partially in a fluid form. Once the composites of the present disclosure have been extruded into a desired shape, the shape may be maintained as consolidation of the thermoplastic polymer(s) occurs.

The at least one thermoplastic polymer may comprise one thermoplastic polymer or more than one thermoplastic polymer. In the event that multiple thermoplastic polymers are present in the polymer material, covalent bonding may take place to each of the various thermoplastic polymers or only some of the thermoplastic polymers. A portion of the thermoplastic polymers, for example, may lack a functional group capable of reacting with a corresponding functional group upon piezoelectric particles to promote formation of a covalent bond between the two.

Optionally, the compositions may further comprise at least one curable resin in combination with the at least one thermoplastic polymer in the polymer material. The at least one curable resin may be uniformly blended with the at least one thermoplastic polymer in the polymer material, or the at least one curable resin may be immiscible with the at least one thermoplastic polymer. The at least one curable resin may be curable by exposure to UV electromagnetic radiation or visible light (photocuring), thermal curing conditions, or any combination thereof to promote covalent crosslinking during or after formation of a printed part. Suitable examples of curable resins will be familiar to one having ordinary skill in the art. Photocuring may promote covalent crosslinking upon a surface portion of the printed part (e.g., up to a depth of about 50 microns or up to about 100 microns, depending on the electromagnetic radiation source used). Thermal curing may promote formation of a covalently crosslinked polymer matrix throughout a printed part, wherein the covalently crosslinked polymer matrix is interblended with the at least one thermoplastic polymer having piezoelectric particles covalently bonded thereto. When at least one curable resin is intended to undergo photocuring, at least one photoinitiator may be present in the compositions disclosed herein. Similarly, when the at least one curable resin is intended to undergo thermal curing, at least one thermal initiator may be present. Suitable examples of photoinitiators and thermal initiators will also be familiar to one having ordinary skill in the art.

Curing of the at least one curable resin may be conducted so as to preclude crosslinking of the at least one thermoplastic polymer. It is to be appreciated, however, that curing may promote internal crosslinking between the piezoelectric particles and/or between the crosslinked polymer and the piezoelectric particles, provided that the piezoelectric particles have been functionalized to include a crosslinkable functional group. Carbon nanomaterials (if present) may similarly be internally crosslinked with themselves and/or with the crosslinked polymer when subjected to suitable curing conditions.

The curable resin may include at least one functional group that may undergo covalent crosslinking under photocuring conditions and/or under thermal curing conditions. Suitable functional groups that may be cured under photocuring or thermal curing conditions may include reactive carbon-carbon double bonds, such as at least one (meth)acrylate compound. As used herein, the term “(meth)acrylate” refers to compound derived from acrylic acid or methacrylic acid, in which a reactive carbon-carbon double bond is attached to the carbon atom adjacent to the acid derivative carbonyl group. Suitable curable resins containing a (meth)acrylate group to promote curing include, for example, (meth)acrylated epoxy monomers or oligomers, (meth)acrylated ester monomers or oligomers, (meth)acrylated urethane monomers or oligomers, (meth)acrylated silicones, aminated (meth)acrylates, halogenated (meth)acrylate monomers or oligomers, aryl (meth)acrylates, aliphatic (meth)acrylates (including aliphatic ring (meth)acrylates), (meth)acrylated ethers, the like, and any combination thereof. Other types of curable functional groups that may be suitable for use in the disclosure herein include, for example, cyclic ethers, vinyl ethers, and certain types of heterocyclic compounds, all of which may be cationically initiated under UV irradiation.

For thermal curing, the curing temperature may be any temperature above room temperature (25° C.); however, the curing temperature may preferably be about 50° C. or higher, or about 60° C. or higher, or about 70° C. or higher, or about 80° C. or higher, or about 90° C. or higher, or about 100° C. or higher, such as about 80° C. to about 120° C. or about 100° C. to about 150° C., or about 120° C. to about 180° C., or about 150° C. to about 200° C.

Optionally, the compositions may further comprise at least one carbon nanomaterial dispersed in at least a portion of the polymer material, such as blended with the at least one thermoplastic polymer in the polymer material. Suitable carbon nanomaterials for use in the disclosure herein may include, for example, exfoliated graphite, exfoliated graphite nanoplatelets, carbon nanofibers (CNF), carbon nanotubes (CNT), graphene, graphene oxides, reduced graphene oxides, graphite oxides, graphene oxide nanosheets, fullerenes, and the like. In particular examples, the carbon nanomaterials within the compositions disclosed herein may comprise a graphene, carbon nanofibers, carbon nanotubes, or any combination thereof. In some examples, the carbon nanomaterials may comprise at least one electrically conductive carbon nanomaterial, optionally in combination with one or more carbon nanomaterials that are not electrically conductive. Illustrative examples of electrically conductive carbon nanomaterials that may be present in the compositions disclosed herein include, for example, single-wall carbon nanotubes, multi-wall carbon nanotubes, carbon nanofibers, graphene, reduced graphene oxide, or any combination thereof. The carbon nanomaterials may be dispersed from one another as individual particles in the compositions disclosed herein. Optionally, the carbon nanomaterials (if present) may be covalently bonded to the at least one thermoplastic polymer and/or to the piezoelectric particles. Optionally, the carbon nanomaterials (if present) may be crosslinked with themselves and/or with piezoelectric particles and/or with a crosslinked polymer after printing of the compositions has taken place.

Suitable graphenes, including graphene oxide, reduced graphene oxide, and the like, may include both single-layer and multi-layer graphene particles. Up to about 10 layers, up to about 20 layers, or up to about 30 layers, or up to about 40 layers, or up to about 50 layers, or up to about 60 layers of sp² hybridized carbon atoms may be present in multi-layer graphenes. Graphene particles may range from about 100 microns to about 1000 microns in size.

Graphene oxide is substantially an electrical insulator and may be produced through chemical exfoliation of graphite. Graphene oxide may be converted to reduced graphene oxide to at least partially restore electrical conductivity thereto. Suitable reducing agents for converting graphene oxide to a reduced graphene oxide include, for example, hydrogen and chemical reducing agents, as will be familiar to one having ordinary skill in the art.

Carbon nanotubes suitable for use in the disclosure herein may include single-wall carbon nanotubes, multi-wall carbon nanotubes or any combination thereof. Single-wall carbon nanotubes may have diameters ranging from about 1 nm to about 10 nm and may have a length to diameter ratio of about 1000 or above, such as about 1000 to about 10000, or about 1000 to about 5000. Multi-wall carbon nanotubes may comprise a plurality of single-wall carbon nanotubes nested within one another and may be up to about 100 nm in diameter for the outermost carbon nanotube.

Carbon nanofibers are larger in diameter than are carbon nanotubes and are usually longer in length. Suitable carbon nanofibers for use in the disclosure herein may include those having diameters ranging from about 100 nm to about 500 nm or about 100 nm to about 300 nm, and lengths ranging from about 5 microns to about 50 microns.

If present, the loading of carbon nanomaterials in the compositions disclosed herein may vary over a wide range, depending on whether the carbon nanomaterials have been introduced to facilitate load transfer or to provide electrical conductivity. When carbon nanomaterials are present in the compositions, the form factors may still remain extrudable and afford printed parts having high structural integrity. Although the electrical conductivity of certain carbon nanomaterials may facilitate poling, an excessive quantity of electrically conductive carbon nanomaterials may be avoided to lessen the likelihood of their promoting dielectric breakdown during the poling process.

When included, carbon nanomaterials may be present in the compositions described herein in an amount up to about 10 vol. % of the composite, or up to about 5 vol. %, or up to about 2 vol. %, or up to 1 vol. %, such as about 0.01 vol. % to about 0.1 vol. %, or about 0.05 vol. % to about 0.5 vol. %, or about 0.1 vol. % to about 0.5 vol. %, or about 0.2 vol. % to about 1 vol. %, or about 0.5 vol. % to about 2 vol. %, or about 1 vol. % to about 3 vol. %, or about 2 vol. % to about 4 vol. %. A maximum volume percentage of the carbon nanomaterials may be selected such that the composites do not experience significant dielectric breakdown during poling of a printed part.

Optionally, the piezoelectric particles and/or the carbon nanomaterials may be covalently bonded with each other, in addition to the piezoelectric particles being covalently bonded to the at least one thermoplastic polymer within the polymer material. Alternately or in addition, the piezoelectric particles and/or the carbon nanomaterials may interact non-covalently with each other and/or with at least a portion of the polymer material. Suitable non-covalent interactions between the piezoelectric particles and/or the carbon nanomaterials and/or the polymer material may include π-π bonding, hydrogen bonding, electrostatic interactions stronger than van der Waals forces, or any combination thereof.

Non-covalent interactions resulting from π-π bonding may arise when two aromatic groups interact interfacially with each other. Thus, to produce a π-π noncovalent interaction between any one or more of the polymer material, the piezoelectric particles, and the carbon nanomaterials, at least one aromatic group may be present upon each component undergoing this type of non-covalent interaction. Non-covalent interactions by π-π bonding can occur when the delocalized π-electron clouds of aromatic ring systems interact interfacially with one another, preferably extended aromatic ring systems containing two or more fused aromatic rings. The aromatic group upon piezoelectric particles undergoing π-π bonding may be directly attached to a surface of the particles or be appended by a linker moiety covalently attached to a surface of the particles. Similarly, carbon nanomaterials may contain reactive surface functionalities (e.g., hydroxyl or carboxylic acid functional groups) to which linker moieties containing appropriate functionality for promoting a non-covalent interaction may be attached by one having ordinary skill in the art. Linker moieties suitable for attaching an aromatic group to piezoelectric particles having hydroxyl groups upon a surface thereof may include, for example, silane-terminated or thiol-terminated linker moieties. Illustrative silane functionalities that can form a covalent bond with surface hydroxyl groups of piezoelectric particles may include, for example, alkoxysilanes, dialkoxysilanes, trialkoxysilanes, alkyldialkoxysilanes, dialkylalkoxysilanes, aryloxysilanes, diaryloxysilanes, triaryloxysilanes, silanols, disilanols, trisilanols, and any combination thereof. Aromatic groups suitable for promoting non-covalent interactions between a polymer material and piezoelectric particles and/or carbon nanomaterials may include, for example, phenyl, naphthalenyl, anthracenyl, phenanthrenyl, pyrenyl, benz(a)anthracenyl, tetracenyl, benzo[a]pyrenyl, benzo[e]pyrenyl, benzo(g,h,i)perylenyl, chrysenyl, and dibenz(a,h)anthracenyl. If not already present in a given type of polymer material, a co-monomer containing an aromatic group may be copolymerized with one or more non-aromatic monomers or grafted onto an existing polymer chain to produce a polymer suitable for promoting π-π bonding. Other types of groups that may bond covalently to the surface of piezoelectric particles for introducing various functionalities thereon, such as an aromatic group, include, for example, phosphines, phosphine oxides, phosphonic acids, phosphonyl esters, carboxylic acids, alcohols, and amines.

Non-covalent interactions resulting from hydrogen bonding may arise when a hydrogen bond donor and a hydrogen bond acceptor interact with each other. The hydrogen bond donor and the hydrogen bond acceptor may be located upon any combination of the piezoelectric particles, the carbon nanomaterials (if present) and the polymer material in the disclosure herein. Hydrogen bond donors may include, for example, hydroxyl groups, amine groups, carboxylic acid groups, and the like. Hydrogen bond acceptors may include any oxygen atom or oxygen-containing functional group, any nitrogen atom or nitrogen-containing functional group, or a fluorine atom. If not already present upon the piezoelectric particles, the carbon nanomaterials, or the polymer material, suitable hydrogen bond donors or hydrogen bond acceptors may be introduced by one having ordinary skill in the art. Optionally, hydrogen bond donors or hydrogen bond acceptors may be introduced onto piezoelectric particles through a linker moiety using similar attachment chemistries to those discussed above.

Non-covalent interactions resulting from electrostatic interactions may arise when any combination of the piezoelectric particles, the carbon nanomaterials (if present), and the polymer material have opposite charges interacting with each other (charge pairing or charge-charge interactions), including induced charge interactions in a dipole. Positively charged groups that may be present upon any of the piezoelectric particles, the carbon nanomaterials, or the polymer material may include, for example, protonated amines and quaternary ammonium groups. Negatively charged groups that may be present upon any of the piezoelectric particles, the carbon nanomaterials, or the polymer material may include, for example, carboxylates, sulfates, sulfonates, and the like. Like other types of non-covalent interactions, suitable groups capable of charge pairing may be introduced upon piezoelectric particles, carbon nanomaterials, or a polymer material by one having ordinary skill in the art, including through attachment of a linker moiety to the piezoelectric particles. Other types of suitable electrostatic interactions may include, for example, charge-dipole, dipole-dipole, induced dipole-dipole, charge-induced dipole, and the like.

The compositions described herein are composites that are extrudable and may be in various form factors. In particular, the polymer material and the piezoelectric particles covalently bonded to at least one thermoplastic polymer (or reactive with at least one thermoplastic polymer) in the polymer material may collectively define a composite having a form factor selected from the group consisting of composite filaments, composite pellets, composite powders, and composite pastes. The piezoelectric particles may be mixed with the polymer material in any of these form factors, such as a substantially uniform dispersion of the piezoelectric particles in at least a portion of the polymer material containing the at least one thermoplastic polymer. For example, when the piezoelectric particles are covalently bonded to the at least one thermoplastic polymer, the piezoelectric particles may be uniformly dispersed in the at least one thermoplastic polymer. If the polymer material comprises a thermoplastic polymer that is incapable of forming covalent bonds with the piezoelectric particles and/or a curable resin that is not crosslinkable with the piezoelectric particles, the piezoelectric particles may be uniformly or non-uniformly dispersed in the polymer material as a whole while still being uniformly dispersed in the at least one thermoplastic polymer to which the piezoelectric particles are covalently bound. Additional description of these form factors follows.

The polymer material or the piezoelectric particles may constitute a majority component of the composites disclosed herein. More preferably, the piezoelectric particles may comprise at least about 10 vol. %, or at least about 20 vol. %, or at least about 30 vol. %, or at least about 40 vol. %, or at least about 50 vol. %, or at least about 60 vol. %, or at least about 70 vol. %, or at least about 80 vol. %, or at least about 85 vol. %, or at least about 90 vol. %, or at least about 95 vol. % of the composites based on total volume. In more particular examples, the piezoelectric particles may comprise about 10 vol. % to about 85 vol. %, or about 25 vol. % to about 75 vol. %, or about 40 vol. % to about 60 vol. %, or about 50 vol. % to about 70 vol. % of the composite. A minimum volume percentage may be selected such that satisfactory piezoelectric properties are realized. A maximum volume percentage of the piezoelectric particles may be chosen such that the composite maintains structural integrity and extrudability. For example, in the case of composite filaments, the amount of piezoelectric particles may be chosen to maintain structural integrity as a continuous filament and that also remains printable by fused filament fabrication. Preferably, the piezoelectric particles may be distributed within the polymer material in a composite under conditions at which the piezoelectric particles remain substantially dispersed as individuals without becoming significantly agglomerated with each other. The piezoelectric particles may be de-agglomerated from each other prior to being combined with a polymer material, as discussed in further detail below.

Composite filaments of the present disclosure may be suitable for use in fused filament fabrication and include a polymer material comprising at least one thermoplastic polymer having a plurality of piezoelectric particles covalently bonded thereto (or reactive therewith to form covalent bonds). Optionally, the polymer material may further comprise at least one curable resin in combination with the at least one thermoplastic polymer. Optionally, at least one polymer precursor, such as a polymerizable monomer or oligomer, may be combined with the at least one thermoplastic polymer in the polymer material. Further optionally, the at least one thermoplastic polymer may comprise one or more thermoplastic polymers that are not covalently bonded to the piezoelectric particles. If present, the at least one curable resin may be curable by exposure to electromagnetic radiation (photocuring), thermal curing conditions, or any combination thereof to promote covalent crosslinking during or after forming a printed part. In non-limiting examples, the composite filaments may be formed by melt blending, preferably such that the piezoelectric particles remain in a substantially non-agglomerated form following formation of the composite filaments. In various embodiments, the piezoelectric particles may be no more agglomerated than an extent of particle agglomeration prior to melt blending. Carbon nanomaterials may similarly be distributed within at least a portion of the polymer material following melt blending and/or remain in a non-agglomerated state.

Composite pellets having distributed piezoelectric particles covalently bonded to at least one thermoplastic polymer therein may similarly be obtained by melt blending, in non-limiting examples. Instead of being produced in an elongate form similar to composite filaments, composite pellets may be characterized by an aspect ratio of about 5 or less and particle sizes having dimensions ranging from about 100 microns to about 5 cm. Composite pellets may feature loadings of piezoelectric particles and other optional components similar to those of composite filaments, and once printed and poled, they may provide a similar range of d₃₃ values. Similarly, the piezoelectric particles may remain in a substantially non-agglomerated form in the composite pellets produced according to the disclosure herein.

Composite powders may be obtained by grinding, milling, pulverizing, or similar processes to produce non-elongate particulates having an irregular shape and a particle size of about 10 microns to about 1 mm, or about 10 microns to about 500 microns, or about 10 microns to about 100 microns.

Extrudable composite pastes may comprise a plurality of piezoelectric particles covalently bonded to at least one thermoplastic polymer comprising a polymer material therein, and a sufficient amount of at least one solvent to promote extrusion at a temperature of interest. The extrudable composite pastes may be monophasic, biphasic, or triphasic. Alternately, extrudable composite pastes may omit a solvent if the polymer material comprises at least one curable resin having a sufficiently low viscosity to facilitate extrusion of the at least one thermoplastic polymer containing covalently bound piezoelectric particles. That is, the at least one curable resin may take the place of a solvent or replace a portion of a solvent when forming an extrudable composite paste according to the disclosure herein. When biphasic or higher, the piezoelectric particles and the polymer material may be present in one or both of the at least two immiscible phases. By virtue of the piezoelectric particles being covalently bonded to the at least one thermoplastic polymer, the piezoelectric particles may be present in at least the same phase where the at least one thermoplastic polymer is located. The polymer material and the piezoelectric particles may be processed into a composite, such as through melt blending and decreasing particle size as discussed above, wherein particles of the resulting pre-made composite are present in at least one phase of the extrudable composite paste. Alternately, a polymer material may be at least partially dissolved or suspended in at least one phase of an extrudable composite paste and piezoelectric particles may be suspended in another place of the extrudable composite paste, such that covalent bonding of the piezoelectric particles to at least a portion of the polymer material may take place upon formulating the extrudable composite paste or as the extrudable composite paste is extruded into a desired shape when forming a printed part.

Optionally, the extrudable composite pastes may comprise a sol-gel material. When present, the sol-gel material may be included in an amount ranging from about 10 wt. % to about 20 wt. %, based on a combined mass of the extrudable composite paste. Inclusion of a sol-gel may result in a stiff matrix following curing, which may enhance the piezoelectric response obtained from the piezoelectric particles.

Suitable solvents that may be present in the extrudable composite pastes may include high-boiling solvents such as, but are not limited to, 1-butanol, 2-methyl-2-propanol, 1-pentanol, 3-methyl-1-butanol, 2,2-dimethyl-1-propanol, cyclopentanol, 1-hexanol, cyclohexanol, 1-heptanol, 1-octanol, propylene carbonate, tetraglyme, glycerol, 2-(2-methoxyethoxy)acetic acid or any combination thereof. Other high-boiling solvents having a boiling point in the range of about 100° C. to about 300° C. may be used as well. Suitable amounts of the at least one solvent may range from about 3 wt. % to about 35 wt. %, based on total mass of the extrudable composite paste.

In some embodiments, the extrudable composite pastes may be biphasic, in which case the at least one solvent may comprise water and a water-immiscible solvent. In non-limiting examples, an aqueous phase may comprise the water, a water-soluble polymer, and an immiscible organic phase may comprise a non-water soluble polymer material and an optional organic solvent. The piezoelectric particles may be covalently bonded to a thermoplastic polymer present within either the aqueous phase or the organic phase. When present, a sol-gel material may be present in the aqueous phase. The water-soluble polymer and the non-water soluble polymer material may be distributed co-continuously with one another, as described in more detail below.

The extrudable composite pastes may exhibit shear-thinning behavior, such that they may be readily extruded but quickly assume a fixed shape having a yield stress of about 100 Pa or greater upon being printed. In non-limiting examples, the extrudable composite pastes may have a viscosity of about 15,000 cP to about 200,000 cP when being sheared at a rate of about 5-10 s⁻¹.

When produced from a suitable form factor, printed parts having good piezoelectric performance may be obtained following printing. More particularly, the composites of the present disclosure may be capable of being printed as a single-layer thin film having a d₃₃ value, after poling, of about 1 pC/N or more at a film thickness of about 200-500 microns, preferably about 200 microns, as measured using an APC International Wide-Range d₃₃ meter. Thin film thicknesses are measured using standard techniques separately from the d₃₃ measurements. In more particular examples, the composites may be capable of forming single-layer thin films having a d₃₃ value, after poling, of about 1 pC/N to about 400 pC/N, or about 2 pC/N to about 200 pC/N, or about 3 pC/N to about 100 pC/N, or about 1 pC/N to about 75 pC/N, or about 5 pC/N to about 50 pC/N, or about 1 pC/N to about 10 pC/N, or about 2 pC/N to about 8 pC/N, or about 3 pC/N to about 10 pC/N, or about 1 pC/N to about 5 pC/N, or about 4 pC/N to about 7 pC/N under these conditions. The loading of piezoelectric particles and suitable blending conditions to maintain the piezoelectric particles as individual particles covalently bonded to the at least one thermoplastic polymer may be selected to afford a desired extent of piezoelectricity. Single-layer film thicknesses that may be printable with the composites may range from about 10 μm to about 500 μm in thickness or about 25 μm to about 400 μm in thickness.

In order to display observable piezoelectric properties, a material such as a printed part or thin film, may be poled. Poling involves subjecting a material to a very high electric field so that dipoles of a piezoelectric material orient themselves to align in the direction of the applied field. Suitable poling conditions will be familiar to one having ordinary skill in the art. In non-limiting examples, poling may be conducted by corona poling, electrode poling or any combination thereof. In corona poling, a piezoelectric material is subjected to a corona discharge in which charged ions are generated and collect on a surface. An electric field is generated between the charged ions on the surface of a material and a grounded plane on the other side of the material. The grounded plane may be directly adhered to the material or present as a grounded plate. In the electrode poling (contact poling), two electrodes are placed on either side of a piezoelectric material, and the material is subjected to a high electric field generated between the two electrodes.

Although poling may be conducted as a separate step, as described above, poling may also be conducted in concert with an additive manufacturing process. In non-limiting examples, a high voltage may be applied between an extrusion nozzle supplying molten composite (formed from the composite filaments or composite pellets disclosed herein) and a grounded plane onto which the molten composite is being deposited to form a printed part.

Suitable piezoelectric particles for use in the present disclosure are not believed to be particularly limited, provided that the piezoelectric particles may be adequately blended with the polymer material and undergo covalent bonding to at least one thermoplastic polymer therein, preferably remaining as individuals once blending with the polymer material has taken place. In the disclosure herein, the piezoelectric particles are covalently bonded to at least one thermoplastic polymer either through a reaction of native functional groups present upon the piezoelectric particles, or the piezoelectric particles may be further functionalized with a linker that attaches a functional group reactive with a compatible functional group located within the at least one thermoplastic polymer (e.g., as a polymer side chain). Suitable examples of functional groups upon the piezoelectric particles and located within the at least one thermoplastic polymer are discussed in further detail below. In one example, surface hydroxyl groups upon the piezoelectric particles may be functionalized with a silane moiety having at least functional group that is reactive with a complementary functional group located within the at least one thermoplastic polymer. Other functionalization strategies for reacting native functional groups upon the surface of piezoelectric particles to introduce a suitable functional group for promoting covalent bond formation may be envisioned by one having ordinary skill in the art. Linker moieties attached to the surface of the piezoelectric particles, such as through the attachment chemistries discussed above, may also be utilized to introduce functional groups capable of forming covalent bonds with a functional group having complementary reactivity upon the at least one thermoplastic polymer.

Illustrative types of covalent bonds that may be formed between the piezoelectric particles and the at least one thermoplastic polymer may include, but are not limited to, ethers, esters, amides, imides, carbon-carbon bonds, metal-ligand bonds, and the like. Other suitable examples will be familiar to one having ordinary skill in the art. Surface functional groups upon the piezoelectric particles and/or functional groups appended to the surface of the piezoelectric particles via a linker moiety may be utilized for forming the covalent bonds.

In non-limiting examples, the functional group upon the piezoelectric particles may comprise one of 1) a nucleophile or 2) an electrophile, and a complementary functional group upon the at least one thermoplastic polymer may comprise the other of 1) a nucleophile or 2) an electrophile. Suitable nucleophiles that may be present upon the piezoelectric particles (either upon the particle surface or bonded through a linker moiety) or upon the at least one thermoplastic polymer may include, for example, alcohols, thiols, amines, carboxylates, and the like. Suitable electrophiles that may be present upon the piezoelectric particles (either upon the particle surface or bonded through a linker moiety) or upon the at least one thermoplastic polymer may include, for example, an alkyl halide, an epoxide, an acyl group (e.g., an aldehyde, a ketone, a carboxylic acid, a carboxylic acid anhydride (including cyclic anhydrides), a carboxylic acid chloride, and the like), an α,β-unsaturated carbonyl, a cyanate, an isocyanate, a thiocyanate, an isothiocyanate, and the like.

In some examples, the piezoelectric particles may include a nucleophile and the at least one thermoplastic polymer may comprise an electrophile that is reactive with the nucleophile upon the piezoelectric particles. In more specific examples, the at least one thermoplastic polymer may comprise a plurality of reactive groups (e.g., upon a side chain or as an end group) comprising a carboxylic acid or a carboxylic acid derivative that is reactive with a nucleophile located upon the piezoelectric particles. Still more specifically, the at least one thermoplastic polymer may comprise a plurality of reactive groups comprising an anhydride, a carboxylic acid, or any combination thereof, and the piezoelectric particles may be covalently bonded to the at least one thermoplastic polymer as a reaction product of at least a portion of the plurality of reactive groups. Suitable nucleophiles that may react with an anhydride group or a carboxylic acid group may include, for example, an amine (e.g., a primary or secondary amine) or an alcohol group. In the case of an amine being present upon the piezoelectric particles, the reaction product may comprise an amide. A cyclic anhydride upon the at least one thermoplastic polymer may form a cyclic imide upon reacting with an amine nucleophile upon the piezoelectric particles. In the case of an alcohol being present upon the piezoelectric particles, the reaction product may comprise an ester when reacted with a carboxylic acid or carboxylic acid derivative upon the at least one thermoplastic polymer.

It is to be recognized that all of the reactive groups upon the at least one thermoplastic polymer need not necessarily undergo a reaction to form a reaction product in the disclosure herein. Accordingly, a thermoplastic polymer that is covalently bonded to a piezoelectric particle may further include a plurality of unreacted reactive groups that have not reacted with piezoelectric particles. The loading of piezoelectric particles in the composites may dictate the extent of the reaction that occurs with the at least one thermoplastic polymer.

Illustrative examples of piezoelectric materials that may be present in piezoelectric particles suitable for use herein include, but are not limited to, crystalline and non-crystalline ceramics, and naturally occurring piezoelectric materials. Suitable crystalline ceramics exhibiting piezoelectric properties may include, but are not limited to, lead zirconate titanate (PZT), potassium niobate, sodium tungstate, Ba₂NaNNb₅O₅, and Pb₂KNb₅O₁₅. Suitable non-crystalline ceramics exhibiting piezoelectric properties may include, but are not limited to, sodium potassium niobate, bismuth ferrite, sodium niobate, barium titanate, bismuth titanate, and sodium bismuth titanate. Particularly suitable examples of piezoelectric particles for use in the disclosure herein may include those containing, for instance, lead zirconate titanate, doped lead zirconate titanate, barium titanate, lead titanate, strontium titanate, barium strontium titanate, lead magnesium niobate, lead magnesium niobate-lead titanate, sodium potassium niobate, calcium copper titanate, bismuth sodium titanate, gallium phosphate, quartz, toumarline and any combination thereof. Suitable dopants for lead zirconate titanate may include, but are not limited to Ni, Bi, La, and Nd.

Other suitable piezoelectric particles may include naturally occurring piezoelectric materials such as, for example, quartz crystals, cane sugar, Rochelle salt, topaz, tourmaline, bone, or any combination thereof. Still other examples of piezoelectric materials that may be used include, for example, ZnO, BiFO₃, and Bi₄Ti₃O₁₂.

The piezoelectric particles employed in the disclosure herein may have an average particle size in a micrometer or nanometer size range. In more particular examples, suitable piezeoelectric particles may have a diameter of about 25 microns or less, or about 10 microns or less, such as about 1 micron to about 10 microns, or about 2 microns to about 8 microns. Smaller piezoelectric particles, such as those having an average particle size under 100 nm or an average particle size of about 100 nm to about 500 nm or about 500 nm to about 1 micron may also be utilized in the disclosure herein. Average particle sizes in the disclosure herein represent D₅₀ values, which refers to a diameter at which 50% of the sample (on a volume basis unless otherwise specified) is comprised of particles having a diameter less than said diameter. D₅₀ may also be referred to as the “average particle size.” Such average particle size measurements may be made by analysis of optical images, including via SEM analysis, or using onboard software of a Malvern Mastersizer 3000 Aero S instrument, which uses light scattering techniques for particle size measurement.

Agglomeration refers to an assembly comprising a plurality of particulates that are loosely held together through physical bonding forces. Agglomerates may be broken apart through input of energy, such as through applying ultrasonic energy, to break the physical bonds. Individual piezoelectric particles that have been produced through de-agglomeration may remain de-agglomerated once blending with a polymer material and covalent bonding to at least one thermoplastic polymer has taken place in the disclosure herein. That is, defined agglomerates are not believed to re-form during the blending processes with a polymer material as disclosed herein. It is to be appreciated that two or more piezoelectric particles may be in contact with one another in a melt-blended piezoelectric composite, but the extent of interaction is less than that occurring in an agglomerate of multiple piezoelectric particulates. In non-limiting examples, agglomerates of piezoelectric particles may have a size ranging from about 100 microns to about 200 microns, and individual piezoelectric particles obtained after de-agglomeration may be in a size range of about 1 micron to about 5 microns or about 1 micron to about 10 microns, or any other size range disclosed above. Particles under 1 micron in size (nanoparticles) may also be obtained in some instances. The de-agglomerated piezoelectric particle sizes may be maintained following formation of a composite having a form factor of the present disclosure.

Thermoplastic polymers suitable for use in the disclosure herein are not believed to be particularly limited, other than allowing piezoelectric particles to be sufficiently dispersed therein and bearing functionality suitable for forming a covalent bond with the piezoelectric particles. Again, covalent bonds may be formed before a suitable form factor of the composites has been deposited to form a printed part, or the covalent bond formation may be latent and occur during or after forming a printed part. In either case, the thermoplastic polymer may be a homopolymer or co-polymer containing at least one functional group suitable for forming a covalent bond with surface functionality upon piezoelectric particles or one or more functional groups appended to the surface of piezoelectric particles via a linker moiety.

In addition to facilitating distribution and covalent bonding of the piezoelectric particles, suitable thermoplastic polymers may exhibit a melting point or softening temperature compatible with extrusion. In non-limiting examples, suitable thermoplastic polymers may exhibit a softening temperature or melting point sufficient to facilitate deposition at a temperature ranging from about 50° C. to about 400° C., or about 70° C. to about 275° C., or from about 100° C. to about 200° C., or from about 175° C. to about 250° C. Melting points may be determined using ASTM E794-06 (2018) with a 10° C. ramping and cooling rate, and softening temperatures may be determined using ASTM D6090-17.

Illustrative examples of suitable thermoplastic polymers may include those commonly employed in fused filament fabrication such as, for instance, a polyamide, a polycaprolactone, a polylactic acid, a poly(styrene-isoprene-styrene) (SIS), a poly(styrene-ethylene-butylene-styrene) (SEBS), a poly(styrene-butylene-styrene) (SBS), a high-impact polystyrene (HIPS), polystyrene, a thermoplastic polyurethane, a poly(acrylonitrile-butadiene-styrene) (ABS), a polymethylmethacrylate, a poly(vinylpyrrolidine-vinylacetate), a polyester (e.g., polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETG), polyethylene naphthalate (PEN), and the like), a polycarbonate, a polyethersulfone, a polyoxymethylene, a polyether ether ketone, a polyether aryl ketone, a polyetherimide, a polyethylene, a polyethylene oxide, a polyphenylene sulfide, a polypropylene, a polystyrene, a polyvinyl chloride, polyphenylene ethers (PPE), a poly(tetrafluoroethylene), a poly(vinylidene fluoride), a poly(vinylidene fluoride-hexafluoropropylene), any copolymer thereof, and any combination thereof.

Other suitable thermoplastic polymers that may be present in the compositions disclosed herein include, for example, polylactic acid, polyvinylpyrrolidone-co-polyvinyl acetate (PVP-co-PVA), copolymers thereof, and the like. Some examples of the composites disclosed herein may include a suitable thermoplastic polymer that is itself non-piezoelectric. In particular examples, some composites disclosed herein may comprise a thermoplastic polymer other than polyvinylidene fluoride.

If not already present in a given type of thermoplastic polymer, a functional group suitable for forming a covalent bond with a complementary functional group upon or appended to the piezoelectric particles may be introduced. When needed for promoting covalent bond formation to the piezoelectric particles, a co-monomer containing the functional group may be copolymerized with another monomer that is incapable of forming covalent bonds to the piezoelectric particles, or the functional group may be grafted onto an existing polymer chain. Accordingly, suitable copolymers for use in the disclosure herein may be any of random copolymers, block copolymers, alternating copolymers, graft copolymers, or any combination thereof. Graft copolymers may include copolymers in which a functional group suitable for reacting with the piezoelectric particles is directly appended to a main polymer chain, indirectly appended to a main polymer chain through a linker moiety defining a branch of the main polymer chain, or present within another polymer chain that is grafted as a branch onto the main polymer chain.

In more specific examples, any of the thermoplastic polymers listed above may include a grafted carboxylic acid or cyclic anhydride group, such as a grafted maleic anhydride group. In some embodiments, the at least one thermoplastic polymer forming a covalent bond to the piezoelectric particles may comprise a maleic anhydride-grafted polyolefin.

In another specific example, the terminal isocyanate groups of a polyurethane polymer may be utilized to form a covalent bond to piezoelectric particles bearing an amine group and/or with native surface functionality upon the piezoelectric particles.

In some embodiments, the polymer material within the composites may comprise a first polymer material and a second polymer material, wherein the first polymer material and the second polymer material are immiscible with each other, such as first and second thermoplastic polymers that are immiscible with one another. In some embodiments, the first polymer material may comprise one or more thermoplastic polymers that are covalently bonded to the piezoelectric particles, and the second polymer material may comprise one or more thermoplastic polymers not covalently bonded to the piezoelectric particles. Alternately, the second polymer material may comprise a curable resin, which may or may not become covalently bonded with the piezoelectric particles upon curing.

A number of benefits and features may be realized by having a first polymer material and a second polymer material that are immiscible with one another. In some instances, the piezoelectric particles and/or carbon nanomaterials may be dispersed in one of the first polymer material or the second polymer material (e.g, the polymer material containing the at least one thermoplastic polymer that is covalently bonded to the piezoelectric particles), thereby effectively increasing the local concentration of the piezoelectric particles and/or the carbon nanomaterials in the composites. In some or other embodiments, one of the first polymer material and the second polymer material may be dissolvable or degradable under specified conditions, whereas the other of the first polymer material and the second polymer material is not. For example, a second polymer material that is not covalently bonded to the piezoelectric particles may be removable therefrom (e.g., through dissolution or degradation). Optionally, non-polymeric materials that are dissolvable or degradable may be utilized to accomplish a similar result. Upon dissolving or degrading one of the first polymer material or the second polymer material (or a non-polymeric dissolvable or degradable material that may be processed into an extrudable composite in combination with a polymer material), a composite having porosity defined therein may be realized. Optionally, the porosity may be backfilled with a material differing from the polymer material (or non-polymeric material) that was removed by degradation or dissolution. If the first polymer material and the second polymer material are distributed co-continuously, as discussed further below, a porous composite having a plurality of interconnected pores may be obtained. Alternately, a first polymer material and a second polymer material, in which neither polymer material is dissolvable or degradable, may be distributed co-continuously, optionally with piezoelectric particles substantially localized in one of the first polymer material or the second polymer material.

When porosity is introduced into a composite of the present disclosure, the porosity may be backfilled with material for further modifying the properties of the composite. In non-limiting examples, the porosity may by backfilled with electrically conductive particles including, but are not limited to, high-conductivity metal particles such as silver, copper, aluminum, gold, and the like; and electrically conductive carbon materials such as carbon black, carbon fibers, graphene, carbon nanotubes, and the like. Illustrative forms for electrically conductive particles may include, for example, nanoparticles, nanoflakes, nanowires, nanorods, microflakes, and the like. Additional materials that may be used for backfilling include, for example, a polymer differing from the polymer that was removed, thermally conductive particles, reinforcement fibers, colorants, stabilizers, plasticizers, the like, and any combination thereof. Such backfilling materials may be introduced in a liquid solution or dispersion, which is subsequently evaporated once backfilling has taken place. Alternately, a low viscosity polymer or low-viscosity curable resin may be utilized instead of a liquid solution or dispersion to promote delivery of the backfilling materials. Carbon nanomaterials, including carbon nanomaterials that are electrically conductive or electrically non-conductive, when introduced into the porosity of a composite, may be the same or different than the carbon nanomaterial combined with the polymer material defining the composite. Moreover, carbon nanomaterials located within the porosity of a composite are differentiated from those combined with the polymer material, since they may be located in the porosity without a polymer material being present in the pores or with a different polymer material being present in the pores.

Other suitable materials for backfilling may include, for example, nanocrystalline cellulose, cellulose nanofibrils, silica, silica-alumina, alumina such as (pseudo)boehmite, gibbsite, titania, zirconia, cationic clays or anionic clays such as saponite, bentonite, kaoline, sepiolite, hydrotalcite, and the like. Other suitable backfilling materials may also include metal oxides such as alumina trihydrate (ATH), alumina monohydrate, magnesium hydroxide, magnesium silicate, talc, silicas such as fumed silica and precipitated silica, and calcium carbonate, calcium metasilicate, Wollastonite, Dolomite, Perlite, hollow glass spheres, kaolin, and the like. UV stabilizers such as titanium oxide, zinc oxide, benzophenones, benzotriazoles, aryl esters, sterically hindered amines, the like, and any combination thereof may also be present.

Composites of the present disclosure may comprise a continuous polymer phase comprising first and second polymer materials, such as first and second thermoplastic polymers, that are immiscible with one another, wherein one of the polymer materials is dissolvable or degradable and may be at least partially removed from the other polymer material under specified conditions, thereby introducing porosity to the composite or printed part formed therefrom. The piezoelectric particles may be distributed within the polymer material that is not removable or degradable by virtue of the covalent bonding to the at least one thermoplastic polymer. The term “continuous polymer phase” refers to the bulk phase in which the piezoelectric particles and optionally carbon nanomaterials are mixed in at least a portion of the polymer matrix. A continuous polymer phase may contain the first and second polymer materials distributed co-continuously or non-co-continuously within one another. In a co-continuous distribution of the first and second polymer materials, the first and second polymer materials may exist as separate, continuous polymer phases that are interblended with each other. That is, the first and second polymer materials may define an interpenetrating network of the polymer materials, wherein there is connectivity between at least a majority of the first polymer material and connectivity between at least a majority of the second polymer material throughout the continuous polymer phase. In a non-co-continuous distribution of the first and second polymer materials, in contrast, isolated pockets of one of the polymer materials may exist in a continuous matrix of the other. Thus, in a co-continuous distribution, any cross-section of the composite contains at least some of both the first polymer material and the second polymer material. Composites containing regions that are separately co-continuous or non-co-continuous also are within the scope of the present disclosure.

When two polymer materials are immiscible, removal of one of the polymer materials may afford controlled porosity or channel introduction into a printed part or a composite form factor used to produce a printed part. Very fine porosity features may be realized, much smaller than those that might be attainable through direct printing. In non-limiting examples, one of the polymer materials may be a water-soluble thermoplastic polymer and the other polymer material may be a water-insoluble thermoplastic polymer. In other instances, one of the polymer materials is dissolvable in an organic solvent and the other polymer material is not soluble in the same organic solvent (but may be soluble in a different organic solvent). In still other instances, one of the polymer materials may be degraded to byproducts that separate from the composite, wherein conditions promoting degradation do not impact the other polymer material. Degradation by melting which removes one of the polymer materials from the other also resides within the scope of the present disclosure. By altering the ratio of the immiscible polymer materials in the composite, the extent of porosity may be regulated to a desired degree. As indicated above, a non-polymeric material that is dissolvable or degradable under specified conditions may similarly be utilized to introduce porosity within a composite form factor or a printed part formed therefrom.

It is to be appreciated that the first polymer material and the second polymer material need not necessarily comprise a single polymer material of each type. Thus, depending on application-specific needs, the first polymer material may comprise one or more polymer materials, such as two or more thermoplastic polymers that are dissolvable or degradable under specified conditions, and the second polymer material may comprise one or more polymer materials, such as two or more thermoplastic polymers that are non-dissolvable or non-degradable under the specified conditions.

Advantageously, piezoelectric particles and/or the carbon nanomaterials may remain substantially associated with or localized in the polymer material that remains undissolved or non-degraded, thus experiencing minimal loss when the dissolvable or degradable polymer material is removed (e.g., through exposure to an appropriate solvent or other conditions that may promote removal of one or more thermoplastic polymers in preference to another). Accordingly, a porous network of the piezoelectric particles and carbon nanomaterials distributed in the remaining polymer material (e.g., an insoluble or non-degradable thermoplastic polymer) may be realized after printing and further processing according to the disclosure herein. The porous network may comprise a plurality of interconnected pores. The piezoelectric particles and the carbon nanomaterials may be uniformly distributed in the remaining polymer material and have a higher effective concentration therein than in the as-produced composite.

Advantageously, composites capable of forming printed parts containing piezoelectric particles and carbon nanomaterials may be formulated using a room temperature aqueous-based process employing a water-soluble polymer. Polyvinyl alcohol (PVA), polyethylene glycol (PEG, also known as polyethylene oxide), or any combination thereof may be combined with piezoelectric particles in an aqueous phase containing a second polymer material (e.g., a second thermoplastic polymer) that is suspended in the aqueous phase and with which the water-soluble polymer is immiscible. After removal of the water from the combined aqueous phase, the two polymer materials may form a continuous polymer phase, in which the two polymer materials remain mutually immiscible with one another, and in which the piezoelectric particles are distributed throughout the continuous polymer phase while being substantially localized in the second polymer material (i.e., the water-insoluble thermoplastic polymer) or at an interface between the first polymer material and the second polymer material. In one example, the continuous polymer phase containing the piezoelectric particles may be obtained as a cast film before being processed into a final form factor, such as those specified above, wherein the piezoelectric particles are substantially localized within the second polymer material in the resulting composite forms and printed parts formed therefrom. Therefore, once formed into a printed part, the first polymer material may be removed to afford a porous polymer network but without releasing substantial quantities of the piezoelectric particles and the optional carbon nanomaterials.

In some examples, the first polymer material may be a water-soluble thermoplastic polymer and the second polymer material may be a water-insoluble thermoplastic polymer. Examples of water-soluble thermoplastic polymers suitable for use in the disclosure herein may include, for example, polyvinyl alcohol, polyethylene glycol, any copolymer thereof, or any combination thereof. Some or other examples of suitable water-soluble thermoplastic polymers may include, but are not limited to, a polyvinylpyrrolidone, a polyoxazoline (e.g., poly(2-ethyl-2-oxazoline)), a cellulose ester, a polylactic acid, a polylactate, a polycaprolactone, any copolymer thereof, or any combination thereof. Solubility or degradation in aqueous acid solutions is also included within the scope of water solubility in the disclosure herein. Polylactic acid may be effectively degraded through contact with an aqueous acid. Polylactic acid may also be used effectively as non-degradable thermoplastic polymer in the disclosure herein (i.e., as a build material), provided that the composite or a printed part formed therefrom is only exposed to conditions chosen so as not to degrade the polylactic acid (i.e., non-acidic conditions). Similar considerations apply to polyesters like polycaprolactones, which may likewise be degradable under aqueous acid conditions but may be suitably used as a second polymer material if the first polymer material is removed under conditions that do not promote their degradation.

When a first polymer material and a second polymer material are combined to form a co-continuous phase, particularly a co-continuous phase containing a dissolvable or degradable thermoplastic polymer and a non-dissolvable or non-degradable thermoplastic polymer, containing piezoelectric particles, the ratio of the first polymer material to the second polymer material may vary over a wide range. In non-limiting examples, a ratio of the first polymer material to the second polymer material may range from about 1:99 to about 99:1 by volume. In more specific examples, the ratio of the first polymer material to the second polymer material may range from about 10:90 to about 90:10, or about 20:80 to about 80:20, or about 30:70 to about 70:30, or about 40:60 to about 60:40, or about 10:90 to about 20:80, or about 20:80 to about 30:70, or about 30:70 to about 40:60, or about 40:60 to about 50:50, or about 50:50 to about 60:40, or about 60:40 to about 70:30, or about 70:30 to about 80:20, or about 80:20 to about 90:10, each on a volume basis. The ratio of the first polymer material to the second polymer material may be selected such that a desired extent of flexibility is realized once a printed part is formed and the first polymer material is removed, or to tailor the extent of porosity formed in a printed part.

After removal of at least a portion of a first polymer material from a printed part, the printed parts may have a degree of porosity commensurate with the amount of the first polymer material that is removed. In non-limiting embodiments, the printed parts may have a porosity ranging from about 5% to about 80%, or about 10% to about 50%, or about 30% to about 70%, based upon the amount of mass removed relative to the total mass of a printed part prior to removal of the first polymer material. The pore size or the channel size of interconnected pores may depend upon the extent of dispersion of the first polymer material in the continuous polymer phase, as well as the amount of the first polymer material that is removed.

Once a first polymer material has been at least partially removed from a printed part, the piezoelectric particles and the optional carbon nanomaterials may be present in a printed polymer matrix comprising the second polymer material (the remaining thermoplastic polymer) at a particle:polymer mass ratio of about 20:80 to about 97:3, based on total mass of the part following removal of the first polymer material. These values correspond to a particle:polymer volume percentage ranging from about 2 vol. % to about 80 vol. %.

By virtue of covalent bonding to one or more thermoplastic polymers therein, at least a majority of the piezoelectric particles may be localized in a polymer material in the continuous polymer phase defining the composite. The extent of localization within the polymer material may be determined based upon the amount of piezoelectric particles lost upon dissolution or degradation of the first polymer material. The piezoelectric particles and optional carbon nanomaterials may be distributed substantially as individuals in the polymer material. In non-limiting examples, at least about 80%, at least about 85%, at least about 90%, or at least about 95% of the piezoelectric particles and optional carbon nanomaterials originally blended with the continuous polymer phase may remain associated with the polymer material once another polymer material has been removed.

In fused filament fabrication processes utilizing composite filaments disclosed herein, the print head may contain one or more extruders, such that a first polymer filament comprising a build material is deposited from a first extruder. The build material may comprise a composite filament in accordance with the disclosure above. Optionally, a second polymer filament comprising a removable support material may be deposited from a second extruder to form a removable support for defining one or more overhangs in a printed part formed from the build material. Filaments (composite filaments or non-composite filaments) suitable for use in the foregoing manner may range from about 0.5 mm to about 10 mm in diameter, or about 1 mm to about 5 mm in diameter, particularly about 1.5 mm to about 3.5 mm in diameter. Standard filament diameters for many three-dimensional printers employing fused filament fabrication technology are 1.75 mm or 2.85 mm (about 3.0 mm). It is to be recognized that any suitable filament diameter may be used in accordance with the disclosure herein, provided that the filament is compatible with a user's particular printing system. Similarly, the length and/or color of the composite filaments is/are not believed to be particularly limited in the printing processes disclosed herein. Preferably, the composite filaments disclosed herein and utilized in processes for forming a printed part are continuous filaments and may be of spoolable length, such as at least about 1 foot, or at least about 5 feet, or at least about 10 feet, or at least about 25 feet, or at least about 50 feet, or at least about 100 feet, or at least about 250 feet, or at least about 500 feet, or at least about 1000 feet. The term “spoolable length” means sufficiently long to be wound on a spool or reel. It is to be appreciated that a composite filament of “spoolable length” need not necessarily be spooled, such as when the composite filament is too rigid to be wound onto a spool.

Accordingly, composite filaments produced according to the disclosure herein may have a diameter and length compatible for use in fused filament fabrication additive manufacturing processes. Particularly suitable examples may include composite filament diameters ranging from about 1 mm to about 10 mm and a length compatible with a continuous printing process. Various filament processing conditions may be utilized to adjust the filament diameter, as explained hereinafter.

Suitable methods for forming composite filaments compatible with fused filament fabrication may take place through melt blending of a polymer material comprising at least one thermoplastic polymer and a plurality of piezoelectric particles combined therewith, which may include melt mixing with stirring, followed by extrusion, or direct extrusion with a twin-screw extruder. The piezoelectric particles may become covalently bonded to the at least one thermoplastic polymer during this process. More specific melt blending methods may comprise combining at least one thermoplastic polymer and a plurality of piezoelectric particles, forming a melt comprising the thermoplastic polymer and the piezoelectric particles, blending the melt, optionally with stirring, to form a melt blend comprising the piezoelectric particles covalently bonded to the at least one thermoplastic polymer and distributed therein, and extruding the melt blend to form a composite filament comprising the piezoelectric particles dispersed in a substantially non-agglomerated form within the at least one thermoplastic polymer. Composite pellets of the present disclosure may be formed in a similar manner, but without extruding directly into a filament form. Instead, the composite may be extruded into a larger diameter fiber that may be cut, shredded, pulverized, ground, or the like to afford composite pellets having a similar morphology to the composite filaments.

In further embodiments, the melt blend may be additionally processed before extruding takes place (e.g., in instances where melt blending takes place prior to extrusion). In particular, the melt blend may be cooled and solidified (e.g., below the melting point or softening temperature of a thermoplastic polymer), and cryogenically milling the melt blend after solidifying and prior to extruding the melt blend. Cryogenic milling will be familiar to one having ordinary skill in the art and may be conducted to reduce the particle size of the melt blend with lower risk of localized heating of the thermoplastic polymer and/or the piezoelectric particles taking place and promoting degradation thereof. Although cryogenic milling may be advantageous, it is to be appreciated that non-cryogenic milling may also be conducted, or the melt blend may be extruded directly without being cooled and solidified first in alternative process variations. Shredding or grinding of the melt blend may also be conducted prior to extrusion as an alternative process variation. In some instances, composite pellets may likewise be obtained without proceeding through a secondary extrusion process.

In some or other embodiments, melt blending methods for forming composites may include de-agglomeration of a piezoelectric material. In particular, piezoelectric particles employed to form the melt blends may be obtained by probe sonication, specifically probe sonication of larger piezoelectric particles or agglomerates thereof, wherein the input of sonic energy promotes de-agglomeration and formation of a reduced particle size. In more specific examples, PZT particles or similar piezoelectric particles processed by probe sonication may have an average particle size of about 10 microns or less, such as a particle size ranging from about 1 micron to about 5 microns, or about 1 micron to about 2 microns. Even smaller piezoelectric particles may be produced in some instances. These piezoelectric particle sizes may be maintained in the composites disclosed herein, with the piezoelectric particles remaining in a substantially non-agglomerated form once blended with a polymer material to define a composite. Other suitable techniques for de-agglomerating piezoelectric particles may include homogenization, ball milling, or the like.

When present, carbon nanomaterials may similarly be de-agglomerated into substantially individual carbon nanomaterial particles by sonication. Other techniques for de-agglomerating carbon nanomaterials will also be familiar to persons having ordinary skill in the art.

In some embodiments, de-agglomerated piezoelectric particles may be reacted with a linker moiety containing a functional group suitable for forming a covalent bond with a complementary functional group upon at least one thermoplastic polymer. In some embodiments, a bridging compound may facilitate covalent bond formation between the piezoelectric particles and the at least one thermoplastic polymer. Suitable bridging compounds may be bifunctional and contain a first functional group that is reactive with the piezoelectric particles and a second functional group that is reactive with the at least one thermoplastic polymer, examples of which will be familiar to persons having ordinary skill in the art. Suitable examples of such complementary functional groups are provided above.

Additive manufacturing processes described herein may comprise providing a composition of the present disclosure, specifically a composite filament, a composite pellet, a composite powder, or a composite paste, and forming a printed part by depositing the composition layer-by-layer. Suitable layer-by-layer deposition techniques will be familiar to one having ordinary skill in the art and may be selected based upon the chosen form of the composite. In non-limiting examples, suitable layer-by-layer deposition techniques may include fused filament fabrication, direct writing, or any combination thereof. When the polymer material comprises at least one thermoplastic polymer, particularly when printing with composite filaments or composite pellets, the composition may be heated at or above a melting point or softening temperature of the at least one thermoplastic polymer when forming the printed part. Thus, once the at least one thermoplastic polymer cools, a printed part having a specified shape may be realized. When the polymer material comprises at least one curable polymer precursor, such as at least one curable resin, forming the printed part may further comprise curing the at least one curable polymer precursor to promote covalent crosslinking thereof. Covalent bond formation may also occur concurrently or after forming a printed part. Curing may take place by, for example, thermal curing or photocuring, following formation of a printed part. Optionally, porosity may be introduced in a printed part, as discussed in further detail above. Poling at least a portion of the printed part may also take place to induce a piezoelectric response therein.

Embodiments disclosed herein include:

A. Compositions comprising piezoelectric particles covalently bonded to a polymer material. The compositions comprise: a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles covalently bonded to the at least one thermoplastic polymer and dispersed in at least a portion of the polymer material; wherein the composition is extrudable.

A1. Printed parts comprising the composition of A.

B. Compositions comprising piezoelectric particles reactive with a polymer material to form covalent bonds. The compositions comprise: a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles dispersed in at least a portion of the polymer material and reactive with the at least one thermoplastic polymer under specified conditions to form a plurality of covalent bonds; wherein the composition is extrudable.

B1. Printed parts comprising the composition of B.

C. Additive manufacturing processes. The processes comprise: providing the composition of A or the composition of B; and forming a printed part by depositing the composition layer-by-layer.

Each of embodiments A, A1, B, B1, and C may have one or more of the following additional elements in any combination:

Element 1: wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite having a form factor selected from the group consisting of a composite filament, a composite pellet, a composite powder, and a composite paste.

Element 2: wherein the piezoelectric particles are uniformly dispersed in at least a portion of the polymer material.

Element 3: wherein the piezoelectric particles comprise about 10 vol. % to about 85 vol. % of the composite.

Element 4: wherein the at least one thermoplastic polymer comprises a plurality of reactive groups comprising an anhydride, a carboxylic acid, or any combination thereof, and the piezoelectric particles are covalently bonded to the at least one thermoplastic polymer as a reaction product of at least a portion of the plurality of reactive groups.

Element 5: wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite filament.

Element 6: wherein the polymer material further comprises at least one curable resin.

Element 7: wherein the piezoelectric particles are substantially non-agglomerated when combined with the polymer material.

Element 8: wherein the piezoelectric particles have an average particle size of about 10 microns or less.

Element 9: wherein the piezoelectric particles comprise a piezoelectric material selected from the group consisting of lead zirconate titanate, doped lead zirconate titanate, barium titanate, lead titanate, strontium titanate, barium strontium titanate, lead magnesium niobate, lead magnesium niobate-lead titanate, sodium potassium niobate, calcium copper titanate, bismuth sodium titanate, gallium phosphate, quartz, tourmaline and any combination thereof.

Element 10: wherein the at least one thermoplastic polymer comprises a first thermoplastic polymer and a second thermoplastic polymer that are immiscible with each other.

Element 11: wherein the piezoelectric particles are substantially localized in and covalently bonded to one of the first thermoplastic polymer or the second thermoplastic polymer.

Element 12: wherein the first thermoplastic polymer and the second thermoplastic polymer collectively define a co-continuous polymer matrix.

Element 13: wherein a plurality of interconnected pores are present in the polymer material.

Element 14: wherein the composition further comprises at least one carbon nanomaterial dispersed in at least a portion of the polymer material.

Element 15: wherein the composition further comprises a bridging compound that makes the piezoelectric particles reactive with the at least one thermoplastic polymer under the specified conditions.

Element 16: wherein the composition is heated at or above a melting point or softening temperature of the at least one thermoplastic polymer when forming the printed part.

Element 17: wherein the polymer material further comprises at least one curable resin, and at least a portion of the printed part is cured by exposure to electromagnetic radiation or thermal curing conditions.

Element 18: wherein the piezoelectric particles are reactive with the at least one thermoplastic polymer under specified conditions, and the piezoelectric particles react to form a plurality of covalent bonds while or after depositing the composition layer-by-layer under the specified conditions.

Element 19: wherein the process further comprises poling at least a portion of the printed part.

Element 20: wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite filament, and forming the printed part comprises a fused filament fabrication process.

By way of non-limiting example, exemplary combinations applicable to A-C and D include, but are not limited to: 1 or 5, and 2; 1 or 5, and 3; 1 or 5, and 4; 1 or 5, and 6; 1 or 5, and 7; 1 or 5, and 8; 1 or 5, and 9; 1 or 5, and 10; 1 or 5, 10 and 11; 1 or 5, 10 and 12; 1 or 5, and 13; 1 or 5, and 14; 1 or 5, and 15; 2 and 3; 2 and 4; 2 and 6; 2 and 7; 2 and 8; 2 and 9; 2 and 10; 2, 10 and 11; 2, 10 and 12; 2 and 13; 2 and 14; 2 and 15; 3 and 4; 3 and 6; 3 and 7; 3 and 8; 3 and 9; 3 and 10; 3, 10 and 11; 3, 10 and 12; 3 and 13; 3 and 14; 3 and 15; 4 and 6; 4 and 7; 4 and 8; 4 and 9; 4 and 10; 4, 10 and 11; 4, 10 and 12; 4 and 13; 4 and 14; 4 and 15; 6 and 7; 6 and 8; 6 and 9; 6 and 10; 6, 10 and 11; 6, 10 and 12; 6 and 13; 6 and 14; 6 and 15; 7 and 8; 7 and 9; 7 and 10; 7, 10 and 11; 7, 10 and 12; 7 and 13; 7 and 14; 7 and 15; 8 and 9; 8 and 10; 8, 10 and 11; 8, 10 and 12; 8 and 13; 8 and 14; 8 and 15; 9 and 10; 9-11; 9, 10 and 12; 9 and 13; 9 and 14; 9 and 15; 10 and 11; 10-12; 10 and 12; 10 and 13; 10, 11 and 12; 10 and 14; 10, 11 and 14; 10 and 15; 10, 11 and 15; 10, 12 and 13; 10, 12 and 14; 10, 12 and 15; 13 and 14; 13 and 15; and 14 and 15.

Clauses of the Disclosure

The present disclosure is further directed to the following non-limiting embodiments:

Clause 1. A composition comprising:

a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles covalently bonded to the at least one thermoplastic polymer and dispersed in at least a portion of the polymer material;

wherein the composition is extrudable. Clause 2. The composition of clause 1, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite having a form factor selected from the group consisting of a composite filament, a composite pellet, a composite powder, and a composite paste. Clause 3. The composition of clause 1 or clause 2, or the composition of clause 1, wherein the piezoelectric particles are uniformly dispersed in at least a portion of the polymer material. Clause 4. The composition of clause 2 or clause 3, or the composition of clause 2, wherein the piezoelectric particles comprise about 10 vol. % to about 85 vol. % of the composite. Clause 5. The composition of any one of clauses 1-4, or the composition of clause 2, wherein the at least one thermoplastic polymer comprises a plurality of reactive groups comprising an anhydride, a carboxylic acid, or any combination thereof, and the piezoelectric particles are covalently bonded to the at least one thermoplastic polymer as a reaction product of at least a portion of the plurality of reactive groups. Clause 6. The composition of clause 1, wherein the at least one thermoplastic polymer comprises a plurality of reactive groups comprising an anhydride, a carboxylic acid, or any combination thereof, and the piezoelectric particles are covalently bonded to the at least one thermoplastic polymer as a reaction product of at least a portion of the plurality of reactive groups. Clause 7. The composition of any one of clauses 1-6, or the composition of clause 1, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite filament. Clause 8. The composition of any one of clauses 1-7, or the composition of clause 1, wherein the polymer material further comprises at least one curable resin. Clause 9. The composition of any one of clauses 1-8, or the composition of clause 1, wherein the piezoelectric particles are substantially non-agglomerated when combined with the polymer material. Clause 10. The composition of any one of clauses 1-9, or the composition of clause 1, wherein the piezoelectric particles have an average particle size of about 10 microns or less. Clause 11. The composition of any one of clauses 1-10, or the composition of clause 1, wherein the piezoelectric particles comprise a piezoelectric material selected from the group consisting of lead zirconate titanate, doped lead zirconate titanate, barium titanate, lead titanate, strontium titanate, barium strontium titanate, lead magnesium niobate, lead magnesium niobate-lead titanate, sodium potassium niobate, calcium copper titanate, bismuth sodium titanate, gallium phosphate, quartz, tourmaline and any combination thereof. Clause 12. The composition of any one of clauses 1-11, or the composition of clause 1, wherein the at least one thermoplastic polymer comprises a first thermoplastic polymer and a second thermoplastic polymer that are immiscible with each other. Clause 13. The composition of clause 12, wherein the piezoelectric particles are substantially localized in and covalently bonded to one of the first thermoplastic polymer or the second thermoplastic polymer. Clause 14. The composition of clause 12 or clause 13, or the composition of clause 12, wherein the first thermoplastic polymer and the second thermoplastic polymer collectively define a co-continuous polymer matrix. Clause 15. The composition of any one of clauses 1-14, or the composition of clause 1, wherein a plurality of interconnected pores are present in the polymer material. Clause 16. The composition of any one of clauses 1-15, or the composition of clause 1, further comprising:

at least one carbon nanomaterial dispersed in at least a portion of the polymer material.

Clause 17. A composition comprising:

a polymer material comprising at least one thermoplastic polymer; and

a plurality of piezoelectric particles dispersed in at least a portion of the polymer material and reactive with the at least one thermoplastic polymer under specified conditions to form a plurality of covalent bonds;

wherein the composition is extrudable.

Clause 18. The composition of clause 17, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite having a form factor selected from the group consisting of a composite filament, a composite pellet, a composite powder, and a composite paste. Clause 19. The composition of clause 17 or clause 18, or the composition of clause 17, wherein the piezoelectric particles are uniformly dispersed in at least a portion of the polymer material. Clause 20. The composition of any one of clauses 17-19, or the composition of clause 17, further comprising: a bridging compound that makes the piezoelectric particles reactive with the at least one thermoplastic polymer under the specified conditions. Clause 21. An additive manufacturing process comprising:

providing the composition of any one of clauses 1-16, of the composition of any one of clauses 17-19, or the composition of clause 1, or the composition of clause 17; and

forming a printed part by depositing the composition layer-by-layer.

Clause 22. The additive manufacturing process of clause 21, wherein the composition is heated at or above a melting point or softening temperature of the at least one thermoplastic polymer when forming the printed part. Clause 23. The additive manufacturing process of clause 22, wherein the polymer material further comprises at least one curable resin, and at least a portion of the printed part is cured by exposure to electromagnetic radiation or thermal curing conditions. Clause 24. The additive manufacturing process of any one of clauses 21-23, or the additive manufacturing process of clause 21, wherein the piezoelectric particles are reactive with the at least one thermoplastic polymer under specified conditions, and the piezoelectric particles react to form a plurality of covalent bonds while or after depositing the composition layer-by-layer under the specified conditions. Clause 25. The additive manufacturing process of clause 24, wherein the composition further comprises a bridging compound that makes the piezoelectric particles reactive with the at least one thermoplastic polymer under the specified conditions. Clause 26. The additive manufacturing process of any one of clauses 21-25, or the additive manufacturing process of clause 21, wherein the piezoelectric particles are substantially non-agglomerated when combined with the polymer material. Clause 27. The additive manufacturing process of any one of clauses 21-26, or the additive manufacturing process of clause 21, wherein the piezoelectric particles have an average particle size of about 10 microns or less. Clause 28. The additive manufacturing process of any one of clauses 21-27, or the additive manufacturing process of clause 21, further comprising:

poling at least a portion of the printed part.

Clause 29. The additive manufacturing process of any one of clauses 21-28, or the additive manufacturing process of clause 21, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite filament, and forming the printed part comprises a fused filament fabrication process. Clause 30. The additive manufacturing process of any one of clauses 21-29, or the additive manufacturing process of clause 21, wherein the composition further comprises at least one carbon nanomaterial dispersed in at least a portion of the polymer material. Clause 31. A printed part comprising the composition of any one of clauses 1-16, or the composition of any one of clauses 17-20, or the composition of clause 1, or the composition of clause 17.

To facilitate a better understanding of the present disclosure, the following examples of preferred or representative embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.

Examples

Lead zirconate titanate (PZT, APC International, Ltd.) was sonicated using a Branson digital probe sonicator for 30 minutes in water at 25% amplitude to break up particle agglomerates. The original PZT agglomerate size of approximately 100 microns afforded PZT particles in a 1-5 micron size range following sonication, with a majority of the PZT particles being in a 1-2 micron size range. The PZT particles were dried at 80° C. overnight in a vacuum oven.

Alternately, a suspension of PZT particles in water (250 g PZT particles in 250 g water) was homogenized under high-shear conditions for 30 minutes using an IKA ULTRA-TURRAX T25 homogenizer. The PZT particles were isolated by centrifugation, washed with isopropanol, and dried at 80° C. in a vacuum oven overnight.

In some instances, the PZT particles were further functionalized upon their surface, as described in further detail below in Example 1.

Further functionalized or native (unfunctionalized) PZT particles were blended with two different polyolefins having a functional group capable of forming covalent bonds with the piezoelectric particles. High density polyethylene (HDPE) with a MFI of 12 g/10 min (190° C./2.16 kg) (Sigma-Aldrich) or linear triblock styrene/ethylene/butylene/styrene copolymer (SEBS) having an MFI of 22 g/10 min (230° C./5 kg) (KRATON G1657 M) were used to prepare comparative piezoelectric composites lacking covalent bonding interactions between the polymer and the piezoelectric particles. HDPE grafted with maleic anhydride (HDPE-g-MA) having a maleic anhydride content of 1.0-1.5 wt. % and also with a MFI of 12 g/10 min (190° C./2.16 kg) (AMPLIFY GR 204, Dow Chemical Company) or a MFI of 4 g/10 min (190° C., 2.16 kg) (Addivant), or SEBS grafted with maleic anhydride (SEBS-g-MA) having a maleic anhydride content of 1.4-2.0 wt. % and also with a MFI of 22 g/10 min (230° C./5 kg) (Kraton Polymer) were used to prepare experimental piezoelectric composites having covalent bonds between the polymer and the piezoelectric particles.

Example 1: Functionalization of PZT Particles with Amino Groups (PZT-NH₂). Functionalization was conducted by combining 50 g PZT with 450 mL isopropanol and 50 mL deionized water in a 1000 mL beaker and sonicating with a probe sonicator in pulse mode with further magnetic stirring for 45 minutes. Pulsing was conducted at 50% amplitude in 1 second intervals, with a 0.5 second interval between pulses. Alternately, de-agglomeration may be conducted as above.

Following sonication, the mixture was transferred to a 1000 mL round bottom flask, and 25 g of aminopropyltriethoxysilane (APTES) was added. The reaction mixture was stirred overnight at room temperature and then heated to 80° C. for 3 hours with magnetic stirring. After cooling to room temperature, the functionalized PZT particles were isolated by centrifugation and washed twice with isopropanol. The functionalized PZT particles (PZT-NH₂) were then dried at 60° C. overnight.

The PZT-NH₂ particles were further characterized by infrared spectroscopy, thermogravimetric analysis, and carbon/hydrogen/nitrogen analyses. FIG. 3 is an illustrative overlay of FTIR spectra of PZT particles and PZT-NH₂ particles. As shown, the FTIR spectrum of unfunctionalized PZT particles was largely featureless, except for a large peak near 510 cm⁻¹, corresponding to metal-O-metal bonds. This peak was maintained in the FTIR spectrum of PZT-NH₂ particles, which also showed ingrowth of several additional smaller peaks at higher wavenumbers. Based upon these analyses, the incorporation of APTES was determined at 3.8 wt. %, based on total mass (16.9 mmol silane/NH₂ per 100 g PZT particles).

Example 2 (Comparative): Formation of a 40 vol. % PZT/HDPE Composite. PZT was combined with HDPE (MFI=12 g/10 min) at 40 vol. %, based on total volume, and the resulting mixture was blended in a Haake compounder at 190° C. Blending was conducted by placing 28.5 g of HDPE pellets in the Haake compounder and melting under mixing conditions. After the HDPE had melted, 153.4 g unfunctionalized PZT particles were then combined with the molten HDPE. After 10-15 minutes of additional blending, the mixture was discharged onto an aluminum pan and cooled to ambient temperature.

Example 3A: Formation of a 40 vol. % PZT/HDPE-g-MA Composite. This composite was prepared as described for Example 2, except HDPE-g-MA (MFI=4 g/10 min) was used.

Example 3B: Formation of a 40 vol. % PZT/HDPE-g-MA Composite. This composite was prepared as described for Example 2, except HDPE-g-MA (MFI=12 g/10 min) was used.

Example 4A: Formation of a 40 vol. % PZT-NH₂/HDPE-g-MA Composite. This composite was prepared as described for Example 2, except HDPE-g-MA (MFI=4 g/10 min) and PZT-NH₂ were used.

Example 4B: Formation of a 40 vol. % PZT-NH₂/HDPE-g-MA Composite. This composite was prepared as described for Example 2, except HDPE-g-MA (MFI=12 g/10 min) and PZT-NH₂ were used.

Example 5 (Comparative): Formation of a 40 vol. % PZT/SEBS Composite. This composite was prepared as described for Example 2, except 27.0 g of SEBS pellets were used, and the blending temperature was 230° C.

Example 6: Formation of a 40 vol. % PZT/SEBS-g-MA Composite. This composite was prepared as described for Example 2, except 27.3 SEBS-g-MA pellets were used, and the blending temperature was 230° C.

Example 7 (Comparative): Formation of a 40 vol. % PZT/Polyurethane Composite. PZT was combined with a thermoplastic polyether polyurethane (ELASTOLLAN 1190A10, BASF) at 40 vol. %, based on total volume, and the resulting mixture was blended in a Haake compounder at 190° C. Blending was conducted by placing 33.9 g of polyurethane pellets in the Haake compounder and melting under mixing conditions. After melting occurred, 153.4 g of PZT particles were combined with the molten polyurethane. After 10-15 minutes of blending, the mixture was discharged onto an aluminum pan and cooled to ambient temperature.

Example 8: Formation of a 40 vol. % PZT-NH₂/Polyurethane Composite (2 wt. % amine loading). This composite was prepared in a similar manner to Example 7, except amine-functionalized PZT (PZT-NH₂) was formed and covalently bonded to unreacted urethane end groups of the polyurethane during blending. After forming molten polyurethane, 3.0 g of (2 wt. % with respect to PZT) of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was combined with the molten polyurethane and mixed for 2 minutes. Thereafter, 153.4 of unfunctionalized PZT particles were added, and after 10-15 minutes of blending, the mixture was discharged onto an aluminum pan and cooled to ambient temperature.

Example 9: Formation of a 40 vol. % PZT-NH₂/Polyurethane Composite (1 wt. % amine loading). This composite was prepared in a similar manner to Example 8, except 1.5 g of (1 wt. % with respect to PZT) of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was combined with the molten polyurethane.

Example 10: Formation of a 40 vol. % PZT-NH₂/Polyurethane Composite (0.5 wt. % amine loading). This composite was prepared in a similar manner to Example 8, except 0.8 g of (0.5 wt. % with respect to PZT) of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane was combined with the molten polyurethane.

The samples from Examples 2-10 are summarized in Table 1 below.

TABLE 1 Polymer PZT Amount Amount Polymer (vol. % PZT (vol. % Bond Type of total) Type of total) Type Example 2 (Comp.) HDPE 60 PZT 40 None Example 3A (Exp.) HDPE-g-MA 60 PZT 40 Covalent (MFI = 4) Example 3B (Exp.) HDPE-g-MA 60 PZT 40 Covalent (MFI = 12) Example 4A (Exp.) HDPE-g-MA 60 PZT- 40 Covalent (MFI = 4) NH₂ Example 4B (Exp.) HDPE-g-MA 60 PZT- 40 Covalent (MFI = 12) NH₂ Example 5 (Comp.) SEBS 60 PZT 40 None Example 6 (Exp.) SEBS-g-MA 60 PZT 40 Covalent Example 7 (Comp.) Polyurethane 60 PZT 40 None Example 8 (Exp.) Polyurethane 60 PZT- 40 Covalent NH₂ Example 9 (Exp.) Polyurethane 60 PZT- 40 Covalent NH₂ Example 10 (Exp.) Polyurethane 60 PZT- 40 Covalent NH₂

FTIR Characterization. Formation of a bonding interaction taking place between the unfunctionalized PZT particles and the cyclic anhydride group of the grafted maleic anhydride was evidenced by FTIR. FIG. 4 is a diagram showing overlaid FTIR spectra of HDPE-g-MA (MFI=12 g/10 min) and the composite of Example 3B. FIG. 5 is a diagram showing overlaid FTIR spectra of SEBS-g-MA and the composite of Example 6. In both cases, the carbonyl peak(s) broadened and shifted to lower wavenumbers in the presence of the unfunctionalized PZT particles, indicating an interaction occurring between the two. The interaction is believed to be a covalent bond resulting from reaction of the anhydride with surface functional groups upon the PZT particles. Direct complexation of a metal ion (e.g., lead) within the PZT particle is also possible.

Formation of a bonding interaction between the functionalized PZT-NH₂ particles and the cyclic anhydride group of the grafted maleic anhydride was also evidenced by FTIR. FIG. 6 is a diagram showing overlaid FTIR spectra of HDPE-g-MA (MFI=12 g/10 min) and the composite of Example 4B. As shown, the carbonyl peaks broadened and shifted to lower wavenumbers in the presence of the PZT-NH₂ particles, indicating a bonding interaction between the two. The peak shifts were much more dramatic than in the case of unfunctionalized PZT particles (Example 3). The peak shifts to lower wavenumbers are believed to be consistent with formation of a cyclic imide or an amide from the cyclic anhydride.

Shore Hardness. Hardness is a measure of the resistance of a material to indentation. Table 2 summarizes the Shore Hardness values of selected samples produced in the examples above. Shore A and Shore D hardness values were determined by a Shore D Rex Model DD-4 instrument according to ASTM S-2240, using the instantaneous instrument readout value.

TABLE 2 Shore A Shore D Entry Sample Hardness Hardness 1 HDPE 96.9 ± 1.1 61.4 ± 0.6 (MFI = 12 g/10 min, Reference) 2 Example 2 — 72.0 ± 0.7 (MFI = 12 g/10 min, Comp.) 3 HDPE-g-MA 97.8 ± 2.0 59.8 ± 0.7 (MFI = 12 g/10 min Reference) 4 Example 3A — 73.0 ± 0.6 (MFI = 4 g/10 min, Exp.) 5 Example 4A — 77.3 ± 1.0 (MFI = 4 g/10 min, Exp.) 6 SEBS (Reference) 57.0 ± 1.1 — 7 Example 5 (Comp.) 79.0 ± 0.6 — 8 SEBS-g-MA (Reference) 79.7 ± 1.6 — 9 Example 6 (Exp.) 88.7 ± 1.1 — As shown, HDPE and HDPE-g-MA reference polymers (Entries 1 and 3) exhibited similar hardness values, while SEBS-g-MA was considerably harder than the SEBS reference material (Entries 6 and 8). Upon incorporating PZT particles within the polymer to form a composite, all hardness values increased. Comparing Entries 4 and 5 against the HDPE reference materials (Entries 1 and 3) and a HDPE-PZT composite lacking the ability to form covalent bonds (Entry 2), covalent bonding increased hardness values still further. Covalent bond formation in Entry 5 appeared to increase the hardness slightly more so than when only unfunctionalized PZT particles were used in Entry 4. Comparing Entries 7 and 9, covalent bonding also increased the hardness of SEBS.

Imaging. The samples produced above were characterized by SEM imaging. FIGS. 7A-7C show illustrative SEM images for HDPE composites produced in Examples 2, 3A, and 4B, respectively. FIG. 7A is an illustrative SEM image for the HDPE composite of Example 2, in which covalent bonds are not possible between the PZT particles and the HDPE. FIGS. 7B and 7C are illustrative SEM images for HDPE-g-MA composites of Examples 3A and 4B, respectively, in which unfunctionalized or functionalized PZT particles undergo covalent bonding with the HDPE-g-MA matrix. In both instances, the PZT particles were more evenly dispersed than when no covalent bonding was present, with functionalized PZT particles being even better dispersed than were unfunctionalized PZT particles. FIGS. 8A and 8B show illustrative SEM images for SEBS composites produced in Examples 5 and 6, respectively. As shown, a more even distribution of PZT particles was observed in SEBS-g-MA compared to unfunctionalized SEBS.

Filament Extrusion. For filament extrusion, the samples were first shredded to afford a coarse powder, and the powder was then extruded using a single-screw Filabot FX6 extruder. The extruder was modified with a digital voltage readout to control the motor speed and extrusion rate. The Filabot EX6 filament extruder is capable of zonal temperature variation among the feed port nozzle, a back zone, a middle zone, and a front zone. The air path of the Filabot EX6 filament extruder may be further adjusted with respect to distance from the feed port nozzle or by raising the air path on a jack. The air path height was kept constant here, and 100% airflow was used during extrusion. Table 3 below summarizes the extrusion conditions and filament properties used when preparing composite filaments from selected samples from above. Measurement of the filament diameter was conducted using an inline thickness gauge.

TABLE 3 Feed Material Example 2 Example 3B Example 6 Source (Comp.) (Exp.) (Exp.) Feed 60° C. 60° C. 40° C. Temperature Back 190° C. 190° C. 190° C. Temperature Middle 190° C. 190° C. 210° C. Temperature Front 170° C. 170° C. 210° C. Temperature Voltage 6.8 V 5.9 V 10 V Nozzle size 2.0 mm 2.0 mm 1.75 mm Air flow 100% 100% 100% Average filament 1.6-1.7 mm 1.6-1.7 mm 1.6-1.7 mm diameter

Printing. The filaments were printed using a Hyrel Hydra 16A 3D printer. Single and multiple layer structures were printed as 2 cm×2 cm squares for evaluation of piezoelectric properties of the composites. Each printed layer was ˜200 μm thick. Printing was performed onto a glass plate coated with a polypropylene release layer. During printing, the extrusion nozzle temperature was maintained at 200° C. and the glass plate was maintained at 100° C.

Piezoelectric Properties. Piezoelectric properties of printed and thermopressed samples were evaluated by measuring d₃₃ values using an APC International Wide-Range d₃₃ meter or a Piezotest PM300 Piezo meter. The d₃₃ meter is capable of measuring d₃₃ values between 1-2000 pC/N at an operating frequency of 110 Hz and an amplitude of 0.25 N. The d₃₃ value represents the quantity of charge generated when a piezoelectric material is subjected to a set applied force (amplitude). Description of the printed and thermopressed samples and their piezoelectric properties are provided in Table 4 below. The samples were thermopressed into 20 mm squares of varying thicknesses using a Carver hydraulic press with the samples heated above their melting point or glass transition temperature in a mold.

Prior to making the d₃₃ measurements, all samples were poled by a corona poling method in which the sample was exposed to a corona discharge for times ranging from 2 to 60 minutes, but more typically 30 minutes. In the corona poling method, the sample was first coated with a thin layer of sputtered metal (Au, Pt, or Al) on one side of the sample, which was then exposed to a wire-generated corona (6-8 kV) located at a distance of about 1 mm from the sample. Since a surface area of approximately 300 μm² is exposed to the corona at a given time, the sample was moved to pole the complete surface through exposure to the corona. The poling process was not optimized. Contact poling in a high dielectric medium may be used as an alternative poling procedure.

TABLE 4 d₃₃ d₃₃ Thickness Thermopressed Printed Entry Composite Formulation (μm) (pC/N) (pC/N) 1 Ex. 2 (Comp.) HDPE (MFI = 12 g/10 100 1.8 — min)/40 vol. % PZT 300 — 2.4 500 0.2 1.2 2 Ex. 3B (Exp.) HDPE-g-MA (MFI = 12 g/10 150 5.3 9.8 min)/40 vol. % PZT 500 — 7.0 3 Ex. 4B (Exp.) HDPE-g-MA (MFI = 12 g/10 400 8.5 — min)/40 vol. % PZT-NH₂ 4 Ex. 5 (Comp.) SEBS/40 vol. % PZT 400 6.5 — 5 Ex. 6 (Exp.) SEBS-g-MA/40 vol. % PZT 400 10.0 — 6 Ex. 7 (Comp.) Polyurethane/40 vol. % PZT 500 5.5 — 7 Ex. 8 (Exp.) Polyurethane/40 vol. % PZT 1000 5.5 — (2 wt. % amine loading) 8 Ex. 9 (Exp.) Polyurethane/40 vol. % PZT 1000 4 — (1 wt. % amine loading) 9 Ex. 10 (Exp.) Polyurethane/40 vol. % PZT 850 6.7 — (0.5 wt. % amine loading)

As shown, unfunctionalized HDPE and SEBS produced relatively low d₃₃ values by virtue of their inability to form covalent bonds to the PZT particles. HDPE-g-MA and SEBS-g-MA composites exhibited higher d₃₃ values than did composites formed from the unfunctionalized polymers, even in thicker (˜500 micron) samples, which are sometimes more difficult to pole. Comparing Entries 2 and 3 against Entry 1, functionalized PZT-NH₂ particles afforded a slightly higher d₃₃ value than did unfunctionalized PZT in composites having comparable thicknesses. In addition, printed samples tended to exhibit higher d₃₃ values than did comparable thermopressed samples. The polyurethane samples (Entries 7-9), all exhibited comparable d₃₃ values to that of the comparative polyurethane sample (Entry 6), in spite of their considerably larger sample thickness.

All documents described herein are incorporated by reference herein for purposes of all jurisdictions where such practice is allowed, including any priority documents and/or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the disclosure have been illustrated and described, various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the disclosure be limited thereby. For example, the compositions described herein may be free of any component, or composition not expressly recited or disclosed herein. Any method may lack any step not recited or disclosed herein. Likewise, the term “comprising” is considered synonymous with the term “including.” Whenever a method, composition, element or group of elements is preceded with the transitional phrase “comprising,” it is understood that we also contemplate the same composition or group of elements with transitional phrases “consisting essentially of,” “consisting of,” “selected from the group consisting of,” or “is” preceding the recitation of the composition, element, or elements and vice versa.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the present specification and associated claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claim, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces.

One or more illustrative embodiments are presented herein. Not all features of a physical implementation are described or shown in this application for the sake of clarity. It is understood that in the development of a physical embodiment of the present disclosure, numerous implementation-specific decisions must be made to achieve the developer's goals, such as compliance with system-related, business-related, government-related and other constraints, which vary by implementation and from time to time. While a developer's efforts might be time-consuming, such efforts would be, nevertheless, a routine undertaking for one of ordinary skill in the art and having benefit of this disclosure.

Therefore, the present disclosure is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to one having ordinary skill in the art and having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present disclosure. The embodiments illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. 

1. A composition comprising: a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles covalently bonded to the at least one thermoplastic polymer and dispersed in at least a portion of the polymer material; wherein the composition is extrudable.
 2. The composition of claim 1, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite having a form factor selected from the group consisting of a composite filament, a composite pellet, a composite powder, and a composite paste.
 3. The composition of claim 2, wherein the piezoelectric particles are uniformly dispersed in at least a portion of the polymer material.
 4. (canceled)
 5. (canceled)
 6. The composition of claim 1, wherein the at least one thermoplastic polymer comprises a plurality of reactive groups comprising an anhydride, a carboxylic acid, or any combination thereof, and the piezoelectric particles are covalently bonded to the at least one thermoplastic polymer as a reaction product of at least a portion of the plurality of reactive groups.
 7. The composition of claim 1, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite filament.
 8. The composition of claim 1, wherein the polymer material further comprises at least one curable resin.
 9. The composition of claim 1, wherein the piezoelectric particles are substantially non-agglomerated when combined with the polymer material.
 10. The composition of claim 1, wherein the piezoelectric particles have an average particle size of about 10 microns or less.
 11. (canceled)
 12. The composition of claim 1, wherein the at least one thermoplastic polymer comprises a first thermoplastic polymer and a second thermoplastic polymer that are immiscible with each other.
 13. The composition of claim 12, wherein the piezoelectric particles are substantially localized in and covalently bonded to one of the first thermoplastic polymer or the second thermoplastic polymer.
 14. (canceled)
 15. (canceled)
 16. (canceled)
 17. A composition comprising: a polymer material comprising at least one thermoplastic polymer; and a plurality of piezoelectric particles dispersed in at least a portion of the polymer material and reactive with the at least one thermoplastic polymer under specified conditions to form a plurality of covalent bonds; wherein the composition is extrudable.
 18. The composition of claim 17, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite having a form factor selected from the group consisting of a composite filament, a composite pellet, a composite powder, and a composite paste.
 19. The composition of claim 18, wherein the piezoelectric particles are uniformly dispersed in at least a portion of the polymer material.
 20. The composition of claim 17, further comprising: a bridging compound that makes the piezoelectric particles reactive with the at least one thermoplastic polymer under the specified conditions.
 21. An additive manufacturing process comprising: providing the composition of claim 1; and forming a printed part by depositing the composition layer-by-layer.
 22. The additive manufacturing process of claim 21, wherein the composition is heated at or above a melting point or softening temperature of the at least one thermoplastic polymer when forming the printed part.
 23. The additive manufacturing process of claim 22, wherein the polymer material further comprises at least one curable resin, and at least a portion of the printed part is cured by exposure to electromagnetic radiation or thermal curing conditions.
 24. (canceled)
 25. (canceled)
 26. The additive manufacturing process of claim 21, wherein the piezoelectric particles are substantially non-agglomerated when combined with the polymer material.
 27. The additive manufacturing process of claim 21, wherein the piezoelectric particles have an average particle size of about 10 microns or less.
 28. (canceled)
 29. The additive manufacturing process of claim 21, wherein the polymer material and the piezoelectric particles collectively define an extrudable material that is a composite filament, and forming the printed part comprises a fused filament fabrication process.
 30. (canceled)
 31. (canceled) 